Complete Monster/Discworld

The Discworld series has seen quite a few Complete Monsters through its run.

""He'd stand there amid the carnage, blood on his hands and stolen jewelry 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""
 * Carcer from the Discworld novel Night Watch is definitely one of these. I can see your house from up here....As Vimes puts it, some people have Good Angel, Bad Angel. Carcer has two competing bad angels.


 * Captain Swing (chief of the Unmentionables). In addition to being the only man who would dream of actually employing a man like Carcer, Swing operates a system in which everyone is guilty by virtue of being suspected. What they are guilty of is merely a fine detail, as is the method of getting them to confess. Swing is arguably more monstrous than Carcer; he is described as a brilliant man, who would never have been employed in the Unmentionables normally, but the torturers and thugs who work for him recognize that in some way, he arrived at brutal thuggishness from the other direction.
 * Jonathan Teatime. Oh dear god...never mind the fact that he's a Self-Made Orphan, what's terrifying 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. Even hardened criminals who've earned their reputation on murder are horrified by him. 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 lockpicker who was an incredibly valuable asset to the Guild. Oh, and his name is pronounced "Te-ah-tim-eh". And don't you ever forget it!
 * Although, contrary to all his other actions, he only gets mildly annoyed when people mispronounce his name.
 * Keep in mind, though, that Teatime is not someone you want to even mildly annoy.
 * 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 full werewolf and, as such, is "impure". Oh, and his favourite game is hunting humans through a forest and 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. Then he messed with Vimes...
 * Small Gods has Vorbis, whose main claim to fame is improving the efficiency of the Quisition. He also encounters a turtle (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:  In the end, Vorbis, who is so self-righteous that he interprets everything around him according to his twisted perception, finds himself in that vast desert and realises that he doesn't believe in anything besides himself, and thus has no afterlife to approach, nothing awaiting him at all. As an indication of his true Messianic tendencies, Brutha, upon his own death, finds Vorbis in this piteous state many years later and offers to guide him across.
 * Also, 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.