Gravity and piano wire; works every time.
Personally I believe in “suicide” chambers. The condemned spends the rest of his life on death row, waiting for his sentence to be executed. During the waiting period, the condemned can place his head against a pane of glass, push a button and a bullet will be discharged into his brain. The chamber is constructed in a way where there can be no accident. Only the condemned can set the suicide in motion.
This way the condemned repents by taking his own life.