Complete Monster: Vorbis. As Death describes him: He was a murderer, and a creator of murderers. A torturer. Without passion. Cruel. Callous. Compassionless. However at the end, Brutha takes pity on him.
However, the Discworld Companion makes it clear that Vorbis is a Knight Templar who did all those horrible things because he genuinely thought they were the right thing to do. In the book it's ambiguous whether he knows he's acting alone and is only in it for power or not.
Magnificent Bastard: Vorbis. He launched his counter-attack before he launched his attack. He sent a man to Ephebe to die to justify the attack, and had to kill him when all they did was mock him.
Also, Lu Tze, who changed the history of Omnia with a compost heap, not sweeping a pile of sand, and a bucket of water.
Moral Event Horizon: Vorbis, being head of the Quisition, seems to have crossed this point well before the start of the book. The death of Brother Murduck happened quite a while ago, and his plan to get through the desert, clearly costing a lot of lives, was even before that.