Father was a marine in Vietnam. Never in my nearly 40 years have I heard him mention the war a single time other than to say he served. I’ve never asked for details as it’s obviously a time he doesn’t want to revisit.
He did his job, and did it well enough to come home intact, both physically and apparently mentally. The fact he struggled with it some merely means he’s a normal moral person, not an amoral monster. Which is a good thing both for him, and for your image of him.
You can wind yourself up into a place where this is a big deal for you. Or you can accept that the world is a messy place full of messy situations where people do messy things.
It’s nice for you that you don’t happen to live in Mogadishu right now. Or Donetsk. Or any number of other nasty places full of nasty violence. But if you had been born there you’d be dealing with all that. And you’d be doing the best you could to stay alive, perhaps by violence yourself.
Thank your stars you’ve got the luxury to consider this stuff from afar, rather than from up close and gut-churningly personal.
Your Dad is / was a good man. Honor him and the sacrifices he made. Combat is neither edifying nor glorious. Nobody is much improved by participating. I wasn’t, nor was anyone I knew. But for all that, nobody deserves to be thought less of by family for answering the call to duty for their country.
Why would what he did in his past affect you terribly?
If you love(d) him then you still feel the same way now except you might realize that he may have done some things that you don’t care for. He’s still your dad.
I can’t say how the killings affected him personally but killing people affects people differently and as smart as The Dope is I’m not sure what help we can be other than recommending possible help for your father, and maybe for you if this affects you. Depending on where you are at of course.
If he was in a NATO force and he was fighting an organization that NATO said were terrorist then it wasn’t murder as far as NATO is concerned.