A girl is climbing a mountain, all by herself, bundled up but short on climbing gear; if she falls she'll hit the ground.
Yeah. He hasn't done anything like that in 300 Years - which is a lot more than that of yours - but he used to do horrible stuff, I guess just because he was unwell and wanted to kill people.
...I guess? I don't know why you'd kill people in the first place so I don't know why you'd decide to stop.
"...so he got caught, but now says he's very sorry, without any actual rehabilitative step in there."
"I mean, sure, but... no explanation besides 'time passed' for why he'd be sorry is kinda..."
"Yeah. They take, like, anger management classes, or find a religion that resonates with them, or make better friends who are good influences on them, or stop drinking, or whatever. Sometimes they just may not care particularly about what they did but prefer not to go back to prison. Sometimes it was a one-off in the first place, crime of passion inspired by someone's incredibly hurtful personal betrayal or something and then that just doesn't happen again. Does any of that sound like what happened?"
The Valar think they can contain him, or they would not have paroled him. So if they can't, it's because they are wrong. And we can't figure that out by asking them.