yeah I don't know what I would do in their situation, I hope I wouldn't degrade into an animal.
That's the thing, it's not a rationalization. It's more, "You can scold me all you want, you would have been doing the exact same thing. You think the 16 year old version of us saw this coming?"
War is ferociously dehumanizing in every way. "...you don't just become a good person again once you hop off the boat and throw away your collection of dried jap scalps." is just the problem. A lot of them are probably very kind hearted people. If they don't tell you the story, you'd confidently say that they are good people. That's why war is so terrible. They are good people. War can make good people turn dead man's skull into a trinket to play with. If you want to maintain your sense of moral superiority, then it doesn't take just keeping yourself in check. It takes an extra-ordinary effort. That's why Eugene's transformation went from kid seeking adventure, to Marine who thinks tearing out a dead soldier's gold tooth is wrong, to being a maggot covered and jaded survivor screaming at his superior "We were all sent here to kill japs weren't we? What the hell difference does it make what weapon we use?!", to being a broken and alcoholic veteran haunted by his experience and what happened to him. War is abusive and horrible, and some groups make sure to dehumanize their own troops to make them do terrible things. Most children aren't comfortable beating and hacking women and children to death, but in Liberia, they did. They did it because the people who manipulated them made sure they were creating monsters from children who were otherwise good.
No one wants to degrade into an animal, but everyone can. Anyone can be driven to that point given enough time and pressure. You're right, it is wrong, hell sometimes it's downright evil. But that's the thing, good people can be made to loose their humanity. Normally that's what leads them to painting their bedroom walls with their brain matter. They really struggle to figure out how they can sometimes get it back. A lot of them don't want to be called out on it because that all happened in a life that's long gone now. They don't feel like the same person who committed those acts. You can see this in some of the prison camps after "Truth and Reconciliation" events happen. People who know what they did, quietly serving their time for participating in a genocide. The society had to come together and accept some punishments that seemed like justice for war and death that really can't be ameliorated.
And that's just the thing, you can call them out, but will you really call everyone out? The Japanese really didn't get punished for liquidating American POW's or experimenting on the Chinese. Then again, LeMay got off from firebombing most of Japan, and no one got charged for dropping Nukes. Will you call our your own friends and family for endorsing the violence, for funding it, for supporting it? Eventually, you'll be sitting by your self on your own self-righteous thrown judging everyone else, right until the horror gets to you too. Then you'll just have to try to survive and live with it like everyone else. Or you could take that .45 and deal with it that way.