I'm not sure why the fact that a pervert who enjoys murdering little boys behaves well when he's locked up without access to young children is a reason to let him out so that he will again have access to our kids. I'm also not sure why it makes sense to announce your plans if you're the father and want to solve the problem permanently after the courts have failed. The dad should wait quietly for his opportunity and then shoot, shovel, and shut up. Problem solved, and I'll bet any decent cop would put more energy into traffic violations and donuts than into solving that disappearance.
He’s probably trying to raise the visibility of this guy getting set free, hoping they’ll reconsider and keep him behind bars.
Afterwards I’d go about my business for a few years, then make my move, if that was my actual intent.
If the dad does kill him, don’t expect juries, the courts, or any government entity to go easy on the guy. The government won’t do it’s duty, but if you try to do it for them, there’ll hell to pay.
The jury might want to help the guy, but their instructions will be so tight that they won’t be able to without endangering themselves.
Good luck dad. You’ve got my blessing.