I think, the answer is obvious.
The death penalty is just pointless. Think of the consequences of not putting a deserving person to the death penalty as opposed to putting an undeserving person to the death penalty. Combined with the high costs and the low effectiveness, it's no surprise that 88% of criminologists disagree with such a punishment.
How many people have been murdered in prison by lifers who should have gotten death, compared to known innocents executed in error?
How many people have been killed by people on parole for killings compared to known innocents executed?
How many scores of millions spent treating the Mansonites to three hots and a cot, and all the prison sex they wanted?