My only thought is that maybe he was never getting out anyway on the other charges, and would prefer to spend his time in state prison as opposed to federal prison. He'd most assuredly go to Oak Park Heights prison because of the high profile nature of his crimes, it's not as bad as some other supermax prisons.
It's also possible that there's some investigations that we aren't privy to that might have led (or he thought they might) eventually lead to the discovery of the body anyway.
It's also possible that there's some investigations that we aren't privy to that might have led (or he thought they might) eventually lead to the discovery of the body anyway.