His mother also told reporters he served three tours of duty in Iraq, and that he is very kind, and not violent.
Because we all know that the Marines value kindness and non-violence, and don't beat it out of grunts in boot camp before teaching them the most effective ways to commit politically sanctioned murder and sending them off to do it.
He served three tours of duty in Iraq. It's virtually guaranteed he saw combat, and odds are very high he has PTSD. Even if he was a kind and non-violent person once upon a time, it is highly doubltful he still is - or even if he is, chances are good those personality traits have been marred or buried under trauma and dehumanization.
War destroys people, physically and mentally. Kind, dutiful young people sign up for the military out of a warped sense of obligation and duty, or a chance to get money for college. Those who genuinely don't want to hurt people are morally placated by empty promises of comfortable deployments in non-combat zones, working desk jobs, not being the one to pull the trigger personally. But the war machine runs on viscera, and the fodder must flow. Their comfortable assignments never materialize, they get handed a tin hat and a gun and sent away to the killing fields.
They come home broken, if they come home at all. Even if they don't catch a bullet, or trip a mine, or get a face full of shrapnel, they still have wounds beneath the flesh, deep in the mind. You can't watch people die and not have it affect you. You can't be part of misery and death and not be touched by it.