I think that at Guantanamo, there were legitimate terrorist prisoners. However, I also think that because of bounties offered and the like, there were many who were sent to Gitmo and held for years and treated to "enhanced interrogation" (i.e. tourture). I don't know about anyone else, but after 3 or 4 years and I'm released because they finally get it that I have no usefull intelligence for them, maybe I'd be a bit resentful and seek out those causing harm to those who'd harmed me.
I'm not condoning this guy's actions, but I gotta at least raise the possibility that this is blowback from our cavalier approach to "the war on terror."
J.M.O. of course.