No longer a threat is not actually true. For one, there is no such thing as life with no chance of release. Any governor can release anyone in his state (I'll admit this is unlikely).
A person given life is still a threat to those around him in the prison. He's no longer a threat to general society, true, but what restraint does he have in prison when he already has the maximum penalty?
There is a LOT of violence in the prisons, to other prisoners and personnel alike. To me it still comes down to the final argument of some crimes deserve the loss of life.