"Agent Orange effects" and "napalm effects." I guess I don't actually regret seeing that stuff, it sure as hell was sobering, but Jesus Christ that was horrifying.
Edit: Not nearly as bad, but similarly relevant and recent. I was watching some combat footage from the Syrian civil war earlier today, a video including a scene of a group of insurgents using a ladder to rush an enemy building. Well, 3 guys go up okay, but one of them gets shot repeatedly in the chest, crumples, and slides down the ladder, where one of his buddies casually tosses the body aside and starts climbing. The violence wasn't nearly as bad as that second guy's nonchalant attitude to his comrade's death. What do you have to go through to be completely unfazed by that?