I disagree, I thinks its a different difference. I don't agree with war either, but at least in that case you are defending your country, War is a case of kill or be killed. If you don't shoot or bomb the guys coming after you, than they will shoot or bomb you.

In the case of a murderer who has already commited the murder, been incarcerated and is no longer a threat to society because he has been separated from society, what is the point of killing him except for outright vengence. Which I would say is more similar to murder than killing someone in self defence or during war.