The negative effects of combat were nightmares, and I'd get jumpy around certain noises and stuff, but you'd have that after a car accident or a bad divorce. Life's filled with trauma. You don't need to go to war to find it; it's going to find you. We all deal with it, and the effects go away after awhile. At least they did for me.
The combat environment has the effect of flattening out civilian identities. If you're young or old, or a graduate from Harvard or the son of a farmer from Alabama, or if you're gay or straight or good-looking or ugly: none of those things matters much in combat, as long as you can conform to the group expectations.
What is and isn't justified by military necessity is, naturally, open to interpretation. One of the key concepts, though, is the law of proportionality. A military attack that results in civilian casualties - 'collateral damage' - is acceptable as long as the military benefits outweigh the price that is paid by humanity.
I don't think people would climb mountains or jump off bridges with parachutes or kayak Class V rapids if those things didn't offer the brief and horrible illusion of imminent death. They would just be complicated, time-consuming endeavors that we'd steer well clear of because they got in the way of real life.