Seems he put his mental health on the line... As someone else here so nicely pointed out, being a drone op isn't an "it's him or me" situation, you're clearly aware that it's just the other guy that's at risk. So if you do have a conscience, this could be a lot harder to deal with than popping a guy who's shooting at you with an AK, at least in a sense in that case you were defending yourself. This guy seems painfully aware of the fact that he bore no risk and he feels guilty for that. It's not shell-shock, but it's the moral equivalent.