Guillotines probably do leave the condemed's brain alive for a good nightmarish half minute till blood loss makes them go unconscious.
Why can't the same procedure used for euthanizing dogs be used on humans? Inject a cocktail of sedative and morphine, wait til they pass out, then inject sodium phenobarbitol. The morphine level needs to be very generous. That said, IMO, about 80 percent of death penalty convictions are unqualified by the crime committed, and with the kind of fuck up stats being brought to life by the woefully underfunded Innocence Project....maybe we need to wait until all those wrongful convictions are cleared up before executing anyone again.