Let him serve out the entirety of his sentence....but please don't pretend that he is somehow worse than other murderers because he killed a famous, or infamous, celebrity.
Lennon was a musician, an entertainer, and a man who disdained conventional mores and was one of the founding fathers of the free-sex movement and the modern drug culture, which have spawned innumerable problems. He didn't deserve to die...but he was no saint.
His murderer deserves no more, and no less, punishment than the murderer of any other ordinary citizen. If he's earned it, he should get the same consideration for parole as any other criminal. If he hasn't earned it, he shouldn't get it.