One can also argue that our Hitman, Number 47, does just about the same thing. Preventing bad people from causing more damage. Where the states and their laws fail, a hitman can still set the world right.
But 47 doesn't mind killing "innocents". It's just that the Agency is usually contacted to kill bad people, but he has killed at least two innocent men in his career (the private investigator trapped in Rendez-Vous in Rotterdam and the mailman in one of Blood Money's cutscenes), and a lot more depending on how you played it. Actually, it bothers me that the game needs excuses for him to kill people. He's a fucking hitman, he's supposed to do "whatever he's payed to do". I wish they just stopped this ridiculous nonsense.