Because killing someone who is guilty is okay. It removes the need to worry about his maintenance or further crimes, thus making easier to focus resources on those things that are really important. And I don't see why you can't let them starve to death. I would have done that with those starting hunger strikes in order to get some benefit or freedom.
And there are murderers who can't change their ways. As I said before, someone able to see the eyes of a woman they are about to rape, hear her cries and not feel the strongest repulse ever, doesn't deserve life, because that man is a monster. And even in some cases of those that may be able to rehabilitate, the risk of them killing another person is too much high to deserve consideration. I prefer to kill someone whose guilt I know and that PERHAPS can be rehabilitated than setting him free after some years to see he is killing or raping once again.
There is a sentence which sums pretty good my feelings on this: if you stab me once, the fault is yours. If you do it twice, the fault is mine. And whereas I agree that religion is a joke, it's not the same to forgive a random wrong that an assassination or rape to someone you love. Nobody should be made to undergo such suffering, provided he has not done it to someone else before. Nobody.
