I mean: come on!
I'd understand not letting him leave when he turned into the smoke monster. But why not allow him to leave when he was human??
Was it his destiny to ALWAYS do something evil? and by keeping him on the island any potential harm he did wouldn't be totally catastrophic to society?
So even if he DID get off the island, maybe he'd become some murderer, warlord, serial killer, a new Hitler? etc etc and possibly destroy society even if he didn't become the smoke monster?
I just thought of the above now, and I like that idea! By keeping him on the island they ensured any evil he did would only be on the island, and not on the whole world!
If that was the case (which could now never be proven) it would explain why he wasn't/couldn't leave back when he was a human.
And if it wasn't - then what is everyone else's explanation for it?
I mean if I was MiB. I'd be pissed off. First his "mother" doesn't let him leave. Then she murders the closest thing he had to a family! Who knows, maybe he had a wife and children who were killed..
Then he stays on the island for like 2,000 years with absolutely no sex whatsoever. That's enough to drive any man crazy. Maybe he wanted to leave the island to go find him some lovin'?