I used to be against it, because I thought most people can be redeemed. Then I got older and wiser. Some people are just evil and their only goal in life is to destroy others too, I think it has to be carefully considered, but if the evidence is overwhelming and it wasn't a mistake or accident and the person is considered a threat to society, the death penalty is cheaper than keeping them alive. Think of people like Osama Bin Laden, Jeffrey Dahmer or Ted Bundy, there is nothing you can do to redeem them, the death penalty also shouldn't be so humane, a firing squad is sufficient in those cases.
I'm an atheist libertarian, but I don't think that matters, I think most people change their views as they get older. Similar why many older people are religious, the current spate of old people were hippies and atheists and communists in the 70s, yet now the majority of the elderly are conservative christians, it's not like all the hippies died, they got older, a real job and had their own children forcing them to become wiser.
I see why too, I believed all religion was bad, but there are definitely long term benefits to having a religious community and even religious rituals.