1) They deserve to die, or
2) They don't deserve to die, but you decide to kill them anyway.
1) If they deserve to die, then they don't deserve a fair fight, because their death is what they should get.
2) If they don't deserve to die, but you decide to kill them anyway, then you're already going after them for bad motives already, so why give them more of a chance to hurt you? You should protect yourself as best you can by fighting "dirty."