In order to make sure that the guillotine will not inflict undue pain to the recipient, we should first test new models on animals with little intelligence and no feelings in order to make sure that it cuts sure and true.
The names Pelosi and Reid come to mind.
I go with suicide as a form of repenting for sin.
Hole in cell wall where bullet will exit into brain. Convict has to create the circuit so there is no accident. Head goes against hole in wall. Two feet on spots on floor, hands on wall create the circuit. From that point on an movement that can open the circuit sets off the charge.
You put the convict in the cell on day one. He will spend the rest of his life in that cell, looking at that hole, realizing what he did. Only then will he realize what he did to take a life.
As a side note, the cell is self cleaning.