The reason Christians should oppose the death penalty is because they believe that (1) humans are Eikons of God who, because of the redemptive work of the trinitarian God in the Cross, Resurrection, and Pentecost, (2) can be restored to union with God and communion with others. Christians can oppose the death penalty because they have hope and believe that God’s grace can undo what has been done and remake the criminal into a person he or she was not previously.
— Read on www.patheos.com/blogs/jesuscreed/2015/05/18/the-death-penalty-american-law-vs-christianity/
And I would add to this:
(3) Time is the means God uses to allow for repentance. Therefore, we should allow for as much time to repent, as is reasonable and possible.