I disagree with your requirement for 100% risk free application of the death penalty. The death penalty has saved thousands of innocent lives in America alone by deterring criminals from murder and effectively eliminating murders who commit murder. To eliminate the death penalty until you are 100% sure in all cases that the legal system is perfect would result in the deaths of 1,000's, just like we saw in the late 60's, 70's and 80's. Victims are also innocent in most cases, and they will die if you require 100% assurance. Life has risks.
Why add the risk of a malfunction in the legal system that might put me to death when it is supposed to protect me? A life sentence in a no-frills work camp prison is deterrence enough for me.