you mean like florida,when old sparky set that guy on fire and he was burning while dying.
and the response in florida was "so?"
The electric chair was a tried and true method, used by several states for many years exactly because it was seen as more humane than hanging. Firing squads can miss vital spots and leave the prisoner to linger for a while. Hangings can go awry, with the same result. The electric chair and, no doubt, lethal injection can also have things go wrong. That's not necessarily cruelty, that's human error.
And that response works for me - "So?"
If you want a no-miss method of execution, have the prisoner seated with two 12-gauge shotguns loaded with buckshot aimed at his brain pan and fire them both at once. That way if one misfires, you still scramble his eggs and he's still a dead sumbitch. But then you get into 'unusual'...