I think people should stop playing this game of good vs evil.
You should be fighting for things you believe in, that is the whole reason for anger to defend what you care about.
You would assassinate someone not just to be a killer, but because you want the government to change, you want new ideas to come through the game and new factions and ideas to become dominant.
You could be killing a king not for another government but maybe even just for the lower social classes. Every King will have things they neglect so there really should always be a reason to want to change who is in charge.
Perhaps you want something more grand, perhaps a different government structure, perhaps change the policies on what is considered crimes.
There is a definite need for assassins to keep the game from being stagnant. Even if you just threaten assasination unless certain changes are made would make sense.