Some US soldiers are sadistic fucks who are only out to murder.
I'd say like 5% of them are like that.
Back in 2003, when the war started, I knew a guy who walked into the Marines recruiting place and said "I want to kill some Iraqis." He was a total Timothy McVay, gun-nut type. Obviously strange and crazy.
But they let him in anyway, and he was part of the assualt on Falluja in 2003. He killed a lot of people there and he loved every minute of it.
Welcome to war. It's not as honorable and pretty as they show you on TV.