I asked this same question in 1997 just as I began my study into the moral justification of killing enemy combatants in war, and the responses were fascinating. Now I hope to learn more, and perhaps to develop a taxonomy of justifications that could become part of Soldiers' professional education. I know how I would justify killing (although I have not killed anyone), but I also realize that there may be other approaches that work for other folks.
I would love to hear from you, if you have killed in war. Your information would remain confidential unless you prefer otherwise.