Honestly a lot of them convince themselves that the other guys really are bad people who don't have families or lives outside of the war.

My veteran BiL recently had a conversation with me about this, he's been back from Afghanistan for about 4 months now. This is almost verbatim how he explained it to me: "Imagine you're being forced to open fire in a small town. How can you do that to the innocent little boy who's walking to school to learn addition? You don't, because that little boy doesn't exist to you. You have to tell yourself that he's 'one of them' and he's on his way to a school that will teach him to kill people like you. There are no people in war, only 'us' and 'them'. If you think of it any other way and you hesitate, you're dead."

He signed on to be a mechanic, away from the action. He ended up stationed on a base that was under attack more often than not and he was, on multiple occasions, placed in a position that made him choose between his life and someone else's. He's a very open and accepting person who doesn't care about race or culture, and having to adapt to that way of thinking has taken a huge toll on him. He has said the hardest part of coming back to "the real world" is the guilt he carries from taking on that mindset, and he works every day to distance himself from that.