I hope the fucker gets everything he deserves. I'm sorry for the family that he destroyed. PTSD is fairly common IME these days, but there's no excuse not to get treated for it if it is affecting your life. Another thing is that if someone is in fact "off" and they are put in a new, stressful environment, even if they never see someone die or kill someone that can be the catalyst to give them that nudge off the edge.
As to your tangent, I'm not sure how well they can prepare soldiers for what they might experience over there. I mean, if you're combat MOS then you'll probably do some form of live fire exercise where rounds are going over your head and obviously you'll be using weapons a lot, but the best idea of what it's like comes from your fellow soldiers who have already been over there and been in the shit. The problem is that you'll listen to all of thier stories, think you know what it's going to be like, and still be mind fucked by it when it happens.
I believe that the army at least is doing a better job of post deployment health treatment. Really, I think that the only way to cut down the numbers we're seeing is to institute psych tests...at least for those who are in combat MOS's. Nothing is worse than getting over there and some fucker who hardly ever leaves the wire finds out his gf is banging his best friend and suddenly he's got "combat stress" and needs to be driven several clicks away to see a dr. a few times a week putting everyone around him at risk. Psych screenings might alleviate that a little bit.
But to come back to the post...that is tragic and I'm sorry to hear that happened.