But he didn't want to die. If you don't want to die -- if you aren't cooperating with the execution -- it is going to be torturous with this method.
The attitude of "this is the way I want to die" is automatically assuming that you want to die. That you're a willing participant. Execution must be presumed to have an uncooperative victim and the humaneness must be judged in that context.
Lot of people saying he should've just let it happen. It would've hurt him less. That's unacceptable to say to a victim of violence in any other context, so I'm not sure why it makes sense to people here.