The Discworld series has seen quite a few Complete Monsters
through its run.
- The Duchess from Wyrd Sisters; understandably, since she's based on Lady Macbeth. When Granny Weatherwax removes the mental walls she put up and shows her all the evil in her own mind, she shrugs it off.
- Small Gods has Vorbis, whose main claim to fame is improving the efficiency of the Quisition. He also encounters a tortoise (not knowing it was secretly the god, Om), turns it over, and wedges it up with stones, and makes a mental note to see what happened in a few hours. Om reflected how different Vorbis would have acted if he knew it was Om, then remembers looking into his mind, and decides that was the sort of person who would put God on his back, just to see what happened. We later find out exactly how Vorbis would act: he tries to kill Om by throwing him overarm into the rocks, not realising he got the wrong tortoise.
- Jonathan Teatime from Hogfather. Lord Downey let him into the Assassinís 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. Even hardened criminals who've earned their reputation on murder are horrified by him. The most terrifying thing about him is that he honestly doesn't understand that his sadistic behaviour is wrong. He sees no difference between having a friendly chat with a man and stabbing him to death
- Wolfgang Von Uberwald from The Fifth Elephant is pretty despicable. Before the book starts, he murders his little sister, Elsa, because she isn't a bimorphic werewolf and, as such, is "impure". He takes the werewolf tradition of hunting humans through a forest and adds the cruel twist of giving them the illusion that they've escaped before ripping them to shreds. He doesn't give a damn about all the political power he and his family can gain through the civil unrest, he just likes being a sadistic bastard.
- Night Watch:
"He'd stand there amid the carnage, blood on his hands and stolen jewellery in his pocket, and with an expression of injured innocence declare, 'Me? What did I do?' And it was believable right up until you looked hard into those cheeky, smiling eyes, and saw, deep down, the demons looking back"
- We also have Carcer's employer: Captain Swing (chief of the Unmentionables).