Should a war story be sad or happy?

I have to write a play for my theatre class and I've made a pretty good plan so far, but I'm not sure how the ending should be, sad or happy. All the main characters are soldiers, this is set in an army training centre and British war camp in Sudan. So far, one out of the four main characters has died. At some point I was planning on one of them, who actually wanted to go into a profession other than soldier, to leave his post and go back to England to follow his dream, but I'm not sure whether he should run away or tell people he's leaving. Also should I kill more people off, or have the rest all return happy? I'm not entirely sure what to do :S