Werewolf transformations. You feel your bones crack and reset, and the failure of your heart, liver and kidneys. You're totally awake for the entire thing, and when you're not screaming, that's only because you can't, because your vocal cords are reshaping. It's explicitly said that humans should simply die from the pain.
George's prequel. He's out on the moor with a slightly obnoxious but well-meaning American tourist trying to find a loch but they get lost, then the American's camera shows that it's a full moon. The two find a sheep that's been torn apart, then run, for the sounds of howling get closer. Eventually the camera is dropped and we hear the two being attacked. When the camera is picked up again and turned on, the tourist is disembowelled, and George is on the floor, bleeding, saying "It got me."
The preference of vampires to work in the police force so they can easily perform cover-ups and lure in their victims. If there's anyone the average British viewer would trust, it's a local policeman, so seeing them repeatedly play on their victims' trust then brutally slaughter people is very creepy.
Mitchell and Daisy slaughtering everyone inside the train car.
Type 4s. Zombies that can feel themselves rotting, and even if they're sliced up into pieces, they're still aware.
George and Eve's lives at the start of series 4. He spends every second, neglecting sleep, watching the door of his baby daughter's room, waiting for vampires to attack. He hangs mirrors around the room to check for if people who enter have reflections, he always carries a mirror and a stake, and he's arranged everything in the room into the shape of crucifixes, including twigs at the third floor window and children's blocks on the floor.
George when he partially transforms in the first episode of Series 4
The possible future shown in Series 4. Vampires have taken over the world, with only a small resistance of humans and werewolves fighting them. London at least has been wiped, along with New York after it. Britons tried to flee the island in boats, but vampires sneaked onto the industrial shippers and slaughtered all aboard. Humans and werewolves were rounded up in camps, which were struck by cholera, and the prisoners were branded to mark them as non-vampires, and in some cases made to fight each other for the vampires' amusement.
Hal has become one of the vampires' leaders, a feared and brutal dictator; Tom was forced to kill others in the camp dog fights and eventually committed suicide in one, and Annie eventually dissolved into the breeze after burying too many friends.
What would have happened if Kirby had succeeded with his plan: an invisible, inaudible serial killer free to murder whoever he wanted, and practically no way of stopping him.
The Old Ones prequel. A cleaner on their convoy ship is shut outside on deck by his crewmates after the captain is told to offer up a sacrifice or everyone aboard would be killed. He then wanders around the freezing deck till he sees a figure ahead in a suit who, when he approaches, shows flashing black eyes and rotting teeth before ripping him apart.
Tom transforming in a club full of young people, slowly advancing towards the crowd. Luckily Hal and Alex get them out.
Cutler's death, knocking down a door after not being invited in and burning alive as he tries to kill baby Eve. Fortunately, or not depending on the bad future, Annie stakes him before he can. All because he wanted to be a history maker. If only his father figure had loved him more. I'm looking at you Hal