Is it justifiable??

Is killing for a good reason justifiable? If you kill someone in your defense, is it okay? what about if you kill someone as a punishment (death sentence)?

In class we compared Raskolnikov killing the pawn broker with the American Revolution. However, when we discussed whether or not murder for a good cause was justifiable, no one brought up Locke. Didn't he say that revolution and attack were okay if you were being oppressed? I'm not saying that I do or don't have an opinion on the subject, because I'm still gathering information so that I can form an opinion. I just found it strange that after ALL the time we spent discussing Locke and his reasoning behind revolution, that not ONE person mentioned his point of view during the discussion.