The death penalty costs more then keeping someone in prison their whole life, which should be a clear indicator to scrap it.
I also feel like it would be 1000x stufftier to rot in prison your whole life than to simply die and get it over with.
I have no religious or moral ties to ending the death penalty, it is just a more cost-effective route. Not to mention the fact that the death penalty seems like too much of an easy way to go in my opinion. Mass-murder tons of people? You could either stay in prison until you die, or just die right now...