(Anti-)war movies tend to portray vets as maladjusted figures. The media tends to accept at face value any homeless guy's claim that he's a war veteran - a claim usually made to get better treatment or just to be able to bask in someone's admiration. The reality is that most homeless people who claim to be war veterans are not. Heck - even respectable people who claim to be war veterans aren't. Going into the military can change people, but it usually helps to discipline them. At the same time, many people who are maladjusted before going in continue being maladjusted after leaving. Joining the military isn't a panacea. By the time someone reaches his majority, he is more or less set in his ways, for good or for ill.
As the daughter of a lifer who served in Viet Nam I have known my share of combat vets in my life. I have never known one to kill anyone, and frankly all seemed like normal well adjusted individuals. I suspect that if you look at stats combat vets are no more likely to kill someone than the public at large.
susie