When Lord Vetinari first appeard in the Discworld books, he was described this way:The current Patrician, head of the extremely rich and powerful Vetinari family, was thin, tall and apparently as cold-blooded as a dead penguin. Just by looking at him you could tell he was the sort of man you'd expect to keep a white cat, and caress it idly while sentencing people to death in a piranha tank; and you'd hazard for good measure that he probably collected rare thin porcelain, turning it over and over in his blue-white fingers while distant screams echoed from the depths of the dungeons. You wouldn't put it past him to use the word 'exquisite' and have thin lips. He looked the kind of person who, when they blink, you mark it off on the calendar. (Sourcery, p. 76)The next paragraph, however, begins: "Practically none of this was in fact the case..." and explains that he owns a small and elderly wire-haired terrier (named Wuffles) and while he occaisionally has people tortured to death, this is considered perfectly acceptable by the overwhelming majority of people. It is worth noting that while everyone in the city who doesn't call him by his first name is rather afraid of him (and some of those who do call him by his first name are, as well), he has never actually killed anybody on-screen, as it were. Oh, references are made, but we readers have yet to see it happen.
Consider: although his Lordship is feared and has all the physical attributes of a villain of the Machiavellian sort (tall, thin, pale, wears black, is very quiet and thoughtful and horrifically well-informed and manipulative), he's improved Ankh-Morpork remarkably and consistently shown himself to be thoroughly dedicated to its welfare.
He is, admittedly, frighteningly practical. While he is against unnecessary violence, he's apparently quite willing to have people killed if there is sufficient need - "dismantling a person is sometimes necessary" (Men At Arms). But he has yet to victimize the truly innocent.* Or even, for that matter, the guilty; while he does have a tendency to give people enough rope to hang themselves and he is perfectly willing to bring up things you wish he wouldn't so that you see the situation from his point of view, he's not, by any stretch of the imagination, a bully. He's simply very practical.
His Lordship may be frightening, he may be unnerving, he may even be, as Sergent Colon so worriedly observed, sarcastic, but he is not a villain. Indeed, I sometimes feel quite strongly that a man like him would do Roundworld here a lot of good.
* Mimes are not innocent. I am all for his policy of stringing them up over scorpion pits.