Well good! Someone has to keep Chapman from continuing to kill John Lennon.
Seriously, though, what's the benefit to society of keeping him in jail? Your personal affinity (or not) for Lennon should have zero sway on whether or not Chapman deserves to remain behind bars.
He's been locked up for long enough to serve as an effective deterrent to copycats, his statements make him seem contrite about his actions, and I don't get the impression that he's an ongoing danger to others.
I don't like the idea of indefinite, punitive incarceration.