It's the "fight or flight" mechanism. Some people will internalize it (self-destructive behavior, depression, suicide) and some people will externalize it (arguing, fighting, killing). None knows how somebody is going to snap. But in a war zone, it's more likely to be externalized and violent.
Again, not defending it and not saying that the guy isn't responsible for his actions. Because we don't live in his head, we can't say for sure what happened to him, so it's better to err on the side of him taking responsibility .
But don't underestimate the effect of enough stress on people. We could all crack under enough pressure and do things that we'd never imagine ourselves doing.
Like it or not, human beings were not mentally and physically built to live with high levels of adrenaline flowing through their veins 24/7 for long periods of time, as they do in a war. That's why we send young people to war. Most of us older people, even the fit ones, would stroke out in less than a year with that kind of stress -- and we older ones have the advantage of knowing who we are and not having that "identity crisis" to deal with that younger people do.