In my fantasy kingdom, a multiple murderer would be placed on a gurney and needled, then wake up in his cell and the next day be placed on a gurney and needled, etc.
He would he told he would be executed up until he reached the number he murdered, maybe sooner, you just never know. When the limit is reached and he wakes up again, he is told the executioners enjoy it so much, they will do it until they tire of the noises he makes as he is placed on the gurney again.