Faith or lack of faith seems irrelevant to the death penalty debate.
If you believe that an eternity of hellfire awaits the guilty, then what does it matter if they spend 30 years in prison before getting there? What's the rush? What's so bad about giving them some time to look forward to getting poked by little red devils with pitchforks forever?
If you don't believe that an eternity of hellfire awaits the guilty, then 30 years in prison is the only punishment they'll ever get.