Myotismon. (Spoilers for the most Wham! Episode filled arcs of the first series ahead.) This guy was the most sadistic and psychopathic villain of the series until Kurata came calling. First, he took in Gatomon under his wings in order to abuse and assault her to the point she became his loyal killing machine. He tortured his Ineffectual Sympathetic Villain minion Demi Devimon for petty reasons, entered the human world and destroyed much of Tokyo, killing several of his hench-mon along the way for the slightest failure (heveryseldom leaves a minion alive, in fact), kidnapped almost every family in Odaiba and forced Gatomon to identify the children of said families as her partner in his search for the Eighth Child or he would kill them all, and killed poor Wizardmon. Any minions he hadn't already killed were cannibalized to fuel his Super Mode, and he was stopped just in time to prevent the entire population of Odaiba from becoming his dessert. The entire time, Myotismon was motivated solely by undying hunger for power over both worlds.
Piedmon is a psychotic Monster Clown who doesn't give a damn about life at all. As the leader and most powerful member of the Dark Masters, supreme leaders of all evil digimon, he is fittingly the most vile member. He led the Dark Masters in all of their destructive acts, including wiping out primary village and all of its' digi eggs entirely (baby killing, pretty much.) Early on he attempts to outright murder Mimi when her crying annoys him, and Chumon ends up Taking the Bullet and dying from being skewered instead. His entire reason for saving himself for last was to enjoy seeing the Digidestined be tormented by his fellow Dark Masters and their minions, and when the Digidestined finally face him down, he savagely beats them down before turning them into living keychains for him to toy with. The sequence where he hunts down the kids and their digimon one by one before transforming them, all while laughing with sadistic glee and mocking the two youngest team members (who he saves for last) is surprisingly scary. But then he actually gets worse! He corners TK and Kari on a rope high above Spiral Mountain. Does he try to kill them with his swords? Nope. Try to turn them into keychains like their friends? Nope. He cuts the rope and sends them plummeting to the ground far below and laughs about it! What makes him so bad is that he's positively giddy about murdering children or confining their souls to a Fate Worse Than Death. Arguably he is just as bad as Myotismon.
Seems the writers are aware of how big of monsters these two are: Piedmon and Myotismon are the only two Digimon Adventure villains who ultimately suffer a Fate Worse Than Death as well. Where Myotismon is ultimately killed Deader Than Dead (having his soul destroyed), Piedmon is sucked into MagnaAngemon's Gate of Destiny, a portal to a dimension from which he can never escape, and Digimon are pretty much immortal baring being killed outright.
While not quite on par with Myotismon or Piedmon in terms of sheer evil, Machinedramon also counts. Most Killer Robots are unfeeling killing machines. Machinedramon's Creepy Monotone would imply this applies to him too. But it's the things he actually says that pushes him into this. Practically everything he says shows he's a cold, calculating sadist who loves killing people, and according to Piedmon, he lives only to destroy. He lent his machine army to Piedmon for his slaughter of the digital guardians in their castle when he went to steal the tags and crests, and his army also wiped out the Otamamon and Gekkomon kingdom, leaving few survivors. It's also indicated that he's the one who killed Piximon by blasting him with his canons. When the DigiDestined are in his territory, he rigs the city with death traps in attempts to kill them, kills his own henchmen when he deems them no longer useful, and massacres an entire hoard of Numemon when they attempt to protect Kari from him. The most chilling line he has in the entire show is telling Kari, a little girl and one of the youngest Digidestined, that he is going to kill her in the most painful way he possibly can. And the combination of his sadistic lines and his Creepy Monotone make almost everything he says Nightmare Fuel. He's not the worst Complete Monster in Digimon, but he certainly makes the list.
The D-Reaper from the Digital World was really just performing its' functionbut in a destructive way. The split D-Reaper that emerges in the real world, however, including J-Reaper and all the agents that follow it, is cold, calculating and intelligent, using Jeri's feelings to come to the conclusion that all humanity is worthless and must be purged. And it attempts to do so in insane and sadistic ways like Mind Rape and Body Horror. It cares nothing for all life, not even it's own artificial existence, making it truly heartless.
If anything, IceDevimon in Digimon Frontier is just as bad, if not worse than the one in Tamers. Here he's an Ax CrazySerial Killer whom even the current Big Bad didn't want anywhere near him, so he kept him in the lowest levels of his fortress to serve as a dungeon. Any other Digimon that ended up their got tortured, frozen, and killed by IceDevimon just for kicks. When the kids end up facing him, he attempts to kill them all, particularly Koichi, who's already in critical condition back in the real world as it is!
Let's not forget laughing hysterically as Ikuto/Keenan sobs over the loss of his surrogate father, whom his Gizumon have just permanently deleted. He is not just the most despicable human villain to ever appear in the Digimon franchise. He is the most despicable VILLAIN to ever appear in the franchise. Period. He even beat out Myotismon, the previous holder of the most complete monster in the series - not an easy feat.
A very good showing of just how horridthis guy is would be the reaction the heroes have to him in the last phase of the final battle. Marcus finally gets ShineGreymon to reach Burst Mode and turns the tables on him. Cue ShineGreymon shooting Kurata one of the most hate-filled Death Glares in anime history while everyone else cheers for Kurata's demise before Marcus and ShineGreymon put Kurata down while he's begging for mercy. Normally, trying to kill someone off who's begging for mercy is the job of the Anti-Hero or in a Big Damn Villains moment, and if the hero does it, it's normally a sign that they've crossed the line. Kurata is such a horrible, evil, terrible excuse for a human being that finishing him off while he's begging for mercy is a completely acceptable action. Basically, Kurata is the digimon equivalent of Doctor Weil.
Lucemon makes another showing in Digimon World Data Squad as The Man Behind the Man for both Tsukasa Kagura and Creepymon in the game's story. The king of the Dark Area and the Chao-Mao Digimon, he's orchestrated everything in the plot by manipulating Kagura's feelings of jealousy to do whatever it took to revive himself. Through him, Lucemon made sure five children throughout the world were merged with one of the Code Keys of the Seven Deadly Sins to create Mao Digimon (The Great Demon Lords). He also has Creepymon attack Kosaburo Katsura and Biyomon to retrieve the Code Keys he's attained. After Creepymon assaults DATS HQ but is defeated, it's revealed Creepymon was made of a former DATS member Masaki Nitta, who was aware of his situation but unable to stop himself, and cannot go back to being human, essentially dying as data before his daughter. When Kagura finally releases the seal on Lucemon, he betrays Kagura and tells him how he deceived the scientist. After absorbing Kagura, Code of Key of Pride himself, Lucemon flees, but not before spreading the influence of the Dark Area. An evil Digimon who deemed himself infinite, immortal, and absolute, Lucemon is a remorseless Omnicidal Maniac who desired to engulf everything in the Dark Area.
Tactimon of Digimon Xros Wars bears special mention in that he was the one leader of Lord Bagra's armies with no positive or redeeming features about him. His honor code is based solely on his standards of perfection rather than true honor and decency. Not only is he responsible for rounding up several good Digimon into labor camps, including Cutemon's parents, but he was prepared to torture Mikey, a child, to death, as opposed to the quick, painless execution he could have ordered (not that that's any better; it just shows how sadistic Tactimon is) and later tries to kill him with a beam canon blast. As if that wasn't bad enough, he's a full follower of the We Have Reserves mantra and has no compunctions about sacrificing his own subordinates for the sake of maintaining his supposedly perfect battle record, a practice that disgusted Dorulumon enough to defect from his side. Towards the end, he uses the negative energy of many Digimon in order to turn their data into dark spirits that he absorbs into his sword in order to boost his own power and ends up taking the life of Grademon without an iota of remorse. Once he got out into the real world, he proceeded to lay waste to the entire area in hopes of getting his enemies killed and taking away what they value. Shortly afterwards, he attempts to murder Angie and Jeremy with his bare hands. When it comes time for him to do battle with his enemies, all Tactimon can truly feel is pure excitement over the thrill of combat and bloodshed.
Another example from the same series is Neo Myotismon, an Expy of Adventure's Myotismon and the second of Lord Bagra's Dark Generals. The ruler of the Vampire Land, he was characterized by only one thing: an undying thirst for power. He possesses familiar spirits known as Evilvils that seek out the life energy of Digimon and give it to him. Not only does he fuse both his minions and his enemies into himself, but he absorbs a great number of innocent Lopmon living in his domain in order to gain immortality from their hidden power. During his battle with the Fusion Fighters, he attempts to absorb Christopher's partner, Metal Greymon, into himself as well, forcing Metal Greymon into an And I Must ScreamBody Horror situation.
The other particularly heinous Dark General is Gravimon, ruler of the Canyon Land. He uses his own core as a way to take Christopher hostage, and then reveals himself as a sadist who uses his cables to torture Christopher and probe his mind. He revels in the following Mind Rape, mercilessly playing off of Christopher's psyche until Christopher becomes broken enough to join him and obey his wishes. This set in motion the events that resulted in, among other things, the tragic death of Deckerdramon, which he ensured by dropping a large gravity attack on top of him. When fighting against the Fusion Fighters, Gravimon tries to physically crush both the kids and their Digimon in his grasp before trying to manipulate Christopher again with a Sadistic Choice - he'd inserted his core into Mikey's body, causing Mikey to endure agonizing pain in the process, and tells Christopher that the only way to destroy both the core and him is to kill Mikey. When Christopher refuses and surrenders instead, Gravimon deems him weak.
Quartzmon is the malicious and omnicidal Big Bad of Digimon Xros Wars the Young Hunters Leaping Through Time, who's responsible for spreading the Digi-Quartz that's destroying the multiverse, manipulated Ryouma into helping him all while using his partner Digimon as a puppet form for himself, absorbs the entire earth into himself, and uses the absorbed spirits of the heroes as a shield so that Tagiru can't attack him. On top of that, he's on D-Reaper levels of nastiness and freakishness.
Played with in regards to Apollomon's Whispered mode and MegaDarknessBagramon. The former was designed to be this trope: Apollomon himself doesn't actually count, but due to the horrific deeds of his ruthless, selfish, and purely malevolent Split Personality, he is perceived as a monster in-universe. The latter becomes one due to Bagramon's gradual Sanity Slippage upon fusing with his brother AxeKnightmon and absorbing all his darkness, making him no more than an Omnicidal Maniac who wants nothing more than to destroy the universe with his power and then rule his new world wielding God-like power over all Digimon and humans who still live within it, despite his claims that he's making an ideal reality, much like Lucemon's delusions of righteousness in Frontier.
C'mon Digimon: Shinichiro Josaki is a vicious bully obsessed with being the strongest Digimon Tamer in the world. As the spoiled heir to Josaki Industries, Shinichiro uses his resources to create the powerful Deathmon, to force children to use their pet-like Digimon to duel him under threat of physical violence. After invariably defeating the children's Digimon, Shinichiro gleefully destroys the Digimon just to torment them before moving on to his next victim. Shinichiro always studies up on his coming opponents so as to learn past traumas or fears and use those to further torture them for his own entertainment, notably trying to burn Kentarou's Digimon alive after learning that Ken once had a pet dog who'd died in a fire. Shinichiro was the first Digimon villain ever created, and still stands out as one of the franchise's most petty and cruel.
Analogman is easily equal to Kurata as being the most vile human villain in the Digimon franchise (to make it clearer, Kurata is an Expy of this guy). A hacker that turned into an Omnicidal Maniac, he came to the Digital World, and sought to rule it by taking over Digimon and turning them into weapons. He saw them as little more than slaves to be used and abused as he saw fit, and says so more than once. The child protagonist, Mameo, fights against him and his partner Machinedramon, and wins. Analogman tries to kill Mameo with a bomb, but he dies instead..only to return as a ghost in Digimon: Digital Card Battle. The player character, heavily implied to be Hiro again (same exact design and all that) comes back to compete in a card tournament that takes a sinister twist when some of the villains from Adventure return. The Final Boss is apparently Venom Myotismon, but he is abruptly hijacked when Analogman, now a ghost, returns and possesses the vampire. Venom Myotismon’s mind is killed, but his body is now used by Analogman as a suit. Analogman challenges Hiro again, and loses, but before he can act on his revenge plan, he is sealed away in the “Deep Net.” Analogman returns again in Digimon World Next Order. Possessing Tsuzuki Shoma, Analogman tries one more campaign of terror against the Digital World, poisoning Digimon all over the world and forcibly turning them into Machinedramon clones. The process is painful, and worse, contagious. While it can be fought, it can also be enhanced by torturing the victims, as Analogman demonstrates more than once. His goal however is to take the Executor, Noir the Omnimon Zwart D, and use his power to destroy a Cosmic Keystone that would erase the entire Digimon multiverse from existence. Why? Just to see what happens next!
An underrated but important example is Millenniummon of the Digimon Wonder Swan Series. He is responsible for the cataclysmic events of the Digimon universe, mainly the Adventureverse, such as releasing Apocalymon. Also, when Milleniummon was destroyed, his data turned into Dark Spores and one buried itself into Ken's neck, exerting its influence on him. Since then, it grew within Ken and was later harvested and implanted into other children by Yukio Oikawa, meaning that Milleniummon is so evil that his data can corrupt anyone and turn them into insane and hateful beings, and Milleniummon is thus indirectly responsible for some of the events in Digimon Adventure 02. Oh, and he was made from the data of Machinedramon. From Bad to Worse really applied here.