So who, in the Discworld canon, would you classify as a "Complete Monster" -- and why? 
Just to be clear: a "Complete Monster" is a character that has no redeeming features and is completely irredeemable. (Needless to say, spoilers galore! Please do not read unless you've read some of the books!)
I'll start ... let's see.
The Duchess from Wyrd Sisters. As the book notes,
Lady Lilith from Witches Abroad?
Jonathan Teatime from Hogfather? The only problem with him is that he is occasionally played for laughs. But
I am not sure about Vorbis (from Small Gods). As horrible as he is,
Wolfgang (from The Fifth Elephant). He
Mr. Pin and Mr. Tulip from The Truth.
Carcer from Night Watch? He is a
Captain Swing from Night Watch? He is
Andy Shank from Unseen Academicals is
I'd say he's not the complete package yet, and hopefully never will be.
What do you think?
I'd be curious to know what you think of Lord Winder, Lord Snapcase, Mr Clete of the Musicians' Guild ... discuss! 
Just to be clear: a "Complete Monster" is a character that has no redeeming features and is completely irredeemable. (Needless to say, spoilers galore! Please do not read unless you've read some of the books!)
I'll start ... let's see.
The Duchess from Wyrd Sisters. As the book notes,
when she is stripped of all of mental barriers she uses to justify her atrocities, she is unaffected because she doesn't mind what she's done. She encourages her husband to kill the King, tried to torture an old woman (not that said old woman was ever in danger) for no real reason, tried to kill the King's infant son, and ordered her husband to kill people who don't pay their taxes. She has the attitude and she's committed several horrible deeds. I'd say she fits.
Lady Lilith from Witches Abroad?
No. She does a lot of horrible things, but Granny Weatherwax (who, being Lilith's sister, has a bias, but is generally speaking, a very just person), doesn't consider her beyond redemption.
Jonathan Teatime from Hogfather? The only problem with him is that he is occasionally played for laughs. But
he has the attitude (sociopath and psychopathic man-child) and he did done some horrible things (it's implied that he killed his parents; he kills anyone who outlives their usefulness so callously that even his subordinates - ruthless criminals - are disgusted; and he kills the Hogfather, which does horrible damage to reality, just because he can).
I am not sure about Vorbis (from Small Gods). As horrible as he is,
Brutha considers him redeemable.
Wolfgang (from The Fifth Elephant). He
killed one of his sisters because she wasn't a full werewolf. His own mother (who is a fairly horrible person anyway) is unnerved by his actions. When his other sister tries to stop his plans, he vows to kill everyone who gets close to her. This sister is tough, but is nevertheless reduced to tears when she realizes how far Wolfgang is going to go to get revenge on her. Vimes, who is very lawful, ultimately decides that Wolfgang is so evil that he turns into a borderline vigilante and executes Wolfgang rather than arrest him. Yep, he's a Complete Monster.
Mr. Pin and Mr. Tulip from The Truth.
Not Mr. Tulip, because he eventually shows remorse for his crimes. As for Mr Pin...he's a horrible person and a weasel, but his worst crime is killing Mr. Tulip to save himself, which doesn't really compare to the others (especially Wolfgang or Carcer).
Carcer from Night Watch? He is a
serial killer who becomes a member of the secret police of a corrupt regime. He makes threats against Vimes's pregnant wife. He unnerves everyone he speaks to. He abuses his subordinates. And he attempts to make Vimes kill him instead of arrest him so he can make Vimes give in to his anger. Yep, he's a Monster.
Captain Swing from Night Watch? He is
head of aforementioned secret police. He tortures people so badly that Vimes has to perform a mercy-killing on some of them. He also kills one of his subordinates for no real reason. He's a Monster, but not as bad as Carcer.
Andy Shank from Unseen Academicals is
a bully and a psychopath, but he never kills anyone.
What do you think?
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