Assassins did have a certain code, after all. It was dishonorable to kill someone if you weren't being paid.

“The trouble with witches is that they’ll never run away from things they really…”

“Being Ymor's right-hand man was like being gently flogged to death with scented…”

“I tell you, commander, it's true that some of the most terrible things in the w…”

“I get it,' said the prisoner. 'Good Cop, Bad Cop, eh?'If you like.' said Vimes.…”