It's not fair that he lived to age 80 and died of natural causes. For a small fraction of criminals, life in prison is not a sufficient punishment, and the death penalty is warranted.
To play devil's advocate, there are two real good reasons for the death penalty:
1. It's the humane thing to do, if we have no choice but to jail someone indefinitely with no hope for redemption.
2. Tax money could be used somewhere else, like improving the lives of the poor and providing mental care services to help people before they get into trouble