I believe that we just borrow our lives from God. And He is the only one who have right to take them away from us.
If you will be based in the bible one of the commandments of the God is We shall not kill. I believe that no one has the right to take human's life we all have a right to live and defend ourselves on accusations even though that person did something wrong they are also humans who has an opportunity to change for better thats why death penalty is not acceptable.