That's not the sole reason. Maybe they are keeping him in because they feel he is still a danger to society. After all, he did kill someone who he claimed to be a fan of?
You don't know the things he says while he's in prison to people, or things he's done. He should stay there until he lives in a coffin and no sooner.
God forbid it ever happens, but if someone murdered someone you love, whether it be a child of yours, or a lover or whatever, you would want that person to feel your pain. You would want them dead, or to serve life imprisonment, where life means life. Just because you may have not felt any pain from John Lennon's death doesn't mean others didn't. He was a very well loved man. All those close to him would want the man to die for what he's done and it's only fair he gets the punishment he deserves.. death.
In some countries, they chop off someone hands if they are convicted of stealing. Or chop off a man's ***** if he is convicted of rape. Things like that, may be harsh but they are deserved because of the crime they have committed. You will never know the pain people go through until it effects you personally.
that is the reason why decisions are made by people unaffected by the crime, people who have been emotionally involved will not make a fair and rational decision.
So you could understand the emotional trauma it would cause for anyone the murder would affect, the outcome of the murder trail is decided by people with no emotional attachment for good reason.