Except that he's most likely never going to be let out.
He sounds more like an idealistic fool at the time. Offended/feeling betrayed at Lennon not practicing what he preached.
If such feelings made him a threat back then, how many other people are there really that he could've been passionate enough about, to object so seriously to their actions? Besides, like I said, seems like he was a kid at the time.
If I personally had to guess, I'd say he's no longer a danger to anyone. 20+ years is a very long time to think and evolve. He's most likely nothing like the person he was then.
... and if we're talking about vague potential threat, I'm sure most of us could be brought to killing another human being given circumstances we individually believed were important enough. So I'd say most of us are potential threats, even if we haven't had reason to yet. I'd almost consider a human incapable of killing another person under any circumstances to be defective. Soldiers fight and kill for causes they believe in... often not even because they believe in the cause, but just because they're being paid to do it.
A guy I knew was having a fight with his girlfriend, and in his frustration and anger, he took out his gun and blew his own head off. He was a pretty cool guy. Emotions make people do stupid things from time to time. Doesn't mean that they'd always do that or that they can't mature with experience.