My wife and I had a discussion about Christians who participate in a war against another country that may have Christians in their army. The question is, will Christians go to heaven after killing other Christians? My wife asked me this many years ago, and I still don't have an answer for her.
Whether war is right or wrong must be examined whether a soldier has the right to kill a soldier from another country. Whether the soldier happens to be a Christian or not, does not change whether the action is morally right or not. The prescription against killing doesn't just apply to Christians.