No, they aren’t, and no, they don’t. Seriously, how many major characters in the Discworld novels have taken a human (or similar) life? Well, okay, Death, but he doesn’t really count. Cohen the Barbarian kills pretty casually, but he’s only been a major character in (IIRC) two books, and his casual violence is usually used as a deliberate contrast to more civilized mores. Vimes has killed a few people, but never casually, and always as part of his duties as commander of the Watch. The distinction is a key point in his characterization, particularly in Thud! and The Fifth Elephant. Excluding Magrat going medieval on a bunch of elves, I don’t recall any of the witches ever actually killing anyone. Nor the wizards, for that matter. Has Susan ever killed anyone? Not counting her stints as Death’s understudy, of course.
So? If they’re licensed, then by definition, it’s not murder.
But seriously, how often has a member of the assassin’s guild been portrayed positively? Vetinari and Teppic from Pyramids are the only names I can come up with off the top of my head, and I don’t think Teppic ever actually killed anyone. And despite his fearsome reputation, I can only think of one time Vetinari actually took someone’s life: when he killed Lord Snapcase in Night Watch. Usually, assassins are villains or at best, stuck-up assholes.
Honestly, I can’t think of a single violent death in any of the Discworld novels that was not treated as a big deal. It’s either a crime that has a major impact on the plot (often being the impetus for it) or it’s a legally and morally justifiable end to the story’s villain. The nearest exception is Susan’s casual slaughter of an incapacitated Auditor in Thief of Time, but by their own defintions, Auditors aren’t people, and anyway, they were trying to destroy all life in the universe at the time, so I think that counts as justifiable homicide.
That’s incorrect. The person who first points out the consequences of Moist’s crimes (which engenders the first twinges of Moist’s conscience) is the golem, Pump.