Assassin

Thugs and bandits kill because people get in the way. Some of them might be good at it, but not the way an assassin dedicate themselves to the kill itself. For an assassin, taking a life is the point of the mission. They might then steal something from the dead person on behalf of their employer - but that's the secondary objective.

Legality

Some societies do have authorised assassins that work for the government. Such places are rarely nice to live in.