IF he was no longer serving, he could have left for a number of reasons... End of enlistment (likely a four year tour, that would fit his age); disability of his own (MIGHT explain the tutu) or some other reason. Not everyone does 20 or 30 years. Some do one tour (enlistment) and leave. Others of us find the Corps a calling, a way of life. Either way is fine. He’s ustill going to be a Marine forever.
What he DID, in this case, was totally wrong and uncalled for. As others have said, probably too much wine coupled with a low flash point. I agree with the victim, community service with disabled vets would be suitable punishment.
Sucker punching a guy in a wheelchair? He needs about 20 years in prison to think about it. I wouldn’t be surprised if something other than alcohol was involved. Steroids, perhaps.