I think whether people join out of desperation and whether it excuses them ethically are two different questions. And the answer to me is, if somebody commits war crimes, the reasons they had for joining don't absolve them of needing to be held accountable for war crimes. Why they joined can matter though, when considering things like: how loyal they are to the military, how interested they are in doing its bidding vs. doing just enough to keep a roof over their head, and so on. To what extent they are a true believer in the military as a force for good vs. someone who could be persuaded otherwise or demoralized easily.

It matters as a strategic question, if nothing else, because any enforcer of the place you live is someone you may have to fight if you're trying to confront the system and do something different, and the less people you have to directly fight, the better.