If you want this in formal terms: "pulling the trigger" and "living with yourself after pulling the trigger" are both skills that some people have and other people don't, those skills exist on a continuum of both directness and effectiveness, and comparative advantage applies all the way down at the level of individuals and their personal skillsets. Even if you support your polity's military aims your polity's military aims may be better accomplished if you aggressively refuse to work on military projects, thereby allowing yourself to contribute more effectively in other areas and freeing others up where they can contribute to military projects more effectively than you could. That's one of the things that tax dollars amortize over, if you're looking at it from high enough up.