I've struggled with this one for years. But, have finally come to peace with it I believe. In my conscience the only time I can see a necessity for this might be serial killers. The legal system is incredibly flawed, especially in relation to punishments fitting the crimes. As more and more evidence comes forward we know that serial killers will kill again, we know that rapists will rape again and we know that molesters will molest again. They cannot be cured and finally the mental health profession is accepting this stance. For a very long time, they believed that rapists and molesters could be treated and cured, they know this is not the case now. Serial killers likewise cannot be cured. Please note in regard to rape I am not talking about the 18 or 20 year old boy convicted of statutory rape. This is an area where the laws are incongruent with the crimes. A boy flashing through the stadium in a steak can be charged with lewd behavior and have to register as a sex offender in many states. A girl can take a photo of herself naked and send it to a boy or boys without their permission or request and he can be charged under pornography laws another sex offense, if he shares it with his buddies, he can be charged with distributing it and if the girl is underage tack on child pornography (we had such a situation more than one actually among the high schoolers in one of our little towns a few years back) In these cases the entire thing is out of whack. Then you get someone who actually is a serial rapist serving 18 months coming out and raping again, because in many places the laws have not been changed in a hundred years.
On the whole however the death penalty requires serious consideration for multiple reasons -- the potential innocence of the person, the ability to protect society in other non-lethal ways, etc.