Although deservedness should certainly be a consideration, it is not the only factor that should decide a person's punishment. In some cases, the criminal justice system must give defendants mercy.
As flawed human beings, we would be in trouble if we got what we deserved all the time. At certain points, everyone has received undeserved forgiveness, which means that we're all obligated to give others the same grace. We must extend criminals the same mercy we have received, and wish to receive, ourselves. For this reason, we should abolish the death penalty.