That's a reasonable argument, but I don't think I could make the decision to work on software that's designed to kill people. I consider myself a pretty good software developer, and yes, I could probably do a better job building software to help kill people than many others, but I don't think "if you don't do it, someone less capable will, and more innocent people will get killed" would sway me. I just don't want to be involved in that, and I'm lucky that I don't have to be.