Wyrd Sisters: Lady Felmet is modeled after Lady Macbeth, only multitudes more sadistic and cruel. The Duchess participated in the overthrow and murder of King Verence I for her husband Leonald to gain the throne for her own benefit and power. Once she inherits her position, she encourages a brutal suppression of peasants and witches alike while using the Fool to spread a propaganda campaign. Revealing herself as a sadist who loves Cold-Blooded Torture, Lady Felmet personally engages in inventive torture with captives. When Granny Weatherwax removes the mental walls she put up and shows her all the evil in her own mind, she shrugs it off. Unlike most Discworld villains, the Duchess is fully aware how evil she is. She just doesn't care.
Hogfather has Jonathan Teatime. Lord Downey let him into the Assassin’s Guild out of pity that his parents died only to suspect that he killed them. If he is sent to kill someone, he will mercilessly slaughter them, their family, and their pets. He kills any accomplices he has the instant they're not currently useful to him, not excepting Ankh-Morpork's most gifted lock-picker who was an incredibly valuable asset to the Guild. He horrifies even hardened criminals who’ve earned their reputation on murder. The most terrifying thing about him is that he honestly doesn't understand that his sadistic behavior is wrong. He sees no difference between having a friendly chat with a man and stabbing him to death
Night Watch: Captain Findthee Swing, the leader of the Unmentionables, is the reason his time period is so corrupt. Swing is a Tautological Templar who uses people's measurement arbitrarily to decide whether someone is good or bad in his eye. He sees himself and his supporters, such as Ax-CrazySerial Killer Carcer Dun, as good and his enemies and victims, like Good Is Not Nice hero Sam Vimes, as evil. When Vimes enters Swing's base, Vimes sees people who have been physically and mentally tortured, one of them is so broken that he does not resist when Vimes kills him without hesitation. Vimes finds Swing about to torture a man tied to a chair, before he comes in to fight Swing to the death.