It's hard to understand because that's not "going all out." https://idioms.thefreedictionary.com/go+all+out

He should just say I'll do what I can while keeping both of us from serious injury. It's like Naruto not using his rasengan while Sasuke uses his chidori wantonly during the kiddy brawl.

Executing prisoners is considered barbaric only if they can't wipe out the entire swarms of heroes once free, and the guy kept saying he's gonna escape openly to everyone able to listen to him in that prison... Pretty sure many governments, including Japan, just covertly snuff out bad elements when they feel the need to. A walking nuke, imprisoned but keeps insisting he'll escape, is a "need to" situation.

The idea I'm putting forth here is there is no real life example of a prisoner like All for One, so it's pointless to compare real world prisoners or laws when dealing with him. Special cases require special measures. That's the safest and most logical thing to do. The guy is an incarnation of evil and has no desire to change, which is generally what prisons are for.

Also, binding him like a roast like that for the rest of his life seems more barbaric than say, lethal injection or a bullet to the head while asleep.