Raskolnikov (Crime and punishment, Dostoevsky) tried that. Didn’t really do his conscience any good.
The problem is, if you have a sense of morality that doesn’t allow killing for wrong reasons, you ll most likely have trouble killing for the right reasons too. It’s just that simple. You are either a cold blooded murderer, a hired killer, or you aren’t. It’s one thing to justify killing someone in your head, but to get the blood on your hands, damn that will fuck up your mind (if you are a thinking person). And the fact that you first want a ‘right reason’ to kill someone, shows that you search for justifications in your actions and that you got a thinking mind. So no, you won’t get away with it. Even if you act upon the intent, which may not really happen, you ll immediately start repenting. Your justifications will start getting weaker, till they totally fade away. In the end, you ll be left with guilt alone and it won’t matter if the reasons for killing were right or wrong.
TL;DR – If you have a conscience, you won’t be able to pull the trigger, no matter the justifications (unless your own life/property/love is at stake). However, if you do go ahead with it, in an impulsive fit, the justifications will lose their meaning and it will either, haunt you for your entire life, or just turn you into a full blown killer.