Sure, if circumstances allow it to happen and if it must be done to protect loved ones, and those not needing of unjust pain.
On one hand hand, selfless murder is about instantaneously exerted power, and it's something that can't be done without bending own psychological barriers. Murder can happen in a rush, instantaneously as adrenaline rushes in fast, and is harder to recall how it had occured rather than how you felt. (Just think of a robber entering your house with you unaware of the robbers presence. Then, you confront the robber. Somehow, you manage to defend yourself.) The consequence of murder can be guilt, or at least the knowledge that what you had done was 'just' if it needed to be done, but nothing more. Or..
On the other hand, murder can be planned, and entirely conscious. There is no doubt that something of this scale has serious repercussions on mental health. You must be mentally conditioned to do something like this. To know that you are going to kill a human being, to know you are going to end someones life is something that can't be avoided in memory. (Just think of the same robber, but you have enough evidence to know that this robbery will occur. So you prepare for the robbers arrival. Another, better example would be child soldiers, as most suffer serious mental and physical repercussions from atrocities of war. Although most don't exactly 'choose' to kill, many child soldiers are more 'forced' to do so to protect others or themselves. The effects on mental health still stands when comparing 'choice' to 'forced'. Both have effects of severe guilt, in most cases.) The effect of knowing you chose to kill someone is a too unbearable thought. It is near impossible to escape the prospect of such a cursed memory. To surpass such thinking and relieve yourself of that kind of stress isn't human; it's inhuman. Stress is what makes part of us who we are.
Stress, our natural instincts and human emotions make us who we are. Human.
Our goal: to escape such stress, give in to our natural instincts, and learn from each others emotions. To kill someone adds more stress, conscious or not, and makes the equation out of balance. That is the effect of murder. Regardless of the outlook on why and for what cause the killing was done, murder is murder.
Unless you are psychopathic. In that case, murder can be an entire different story!.. But enough of this. By now, you should see my point on what I think of murder already.