It sounds like if you had it your way, every kid that ever terrorized an animal (I knew a lot of them when I was a kid) would be put in jail for life because of what they could become.
Let's just try to sit back and wait it out a little. If he did in fact kill a puppy, the military has several options including, from an article on Google "It's all governed by the Uniform Code of Military Justice, and anything from reduction in pay to reduction in rank to confinement to discharge from the Marine Corps" is possible, Perrine said. If others are involved, they too will be disciplined, he said."
Do not think for a second that being discharged, especially if it's dishonorably, is going to be a pick nick for this guy. If they do that, his life will essentially be ruined. Every time he fills out an application, he'll have to explain (assuming he even gets an interview) why he was dishonorably discharged. He's going to have a VERY tough time finding any work if this turns out to be true.
I know how easy it is to edit video though. I've done some edits myself where you'd never know it was two different pieces of video. This could all turn out to be a fake.