Serious question: What’s the objection to a gas chamber that fills up with carbon monoxide instead of the fancy stuff? That “works” in any number of painfully hard to read articles about faulty kerosene heaters every winter. I’m of the understanding that you just kinda fall asleep and quietly and painlessly snuff it.
I’d just as soon it was slow, painful and terrifying.
Just turn the friends and family of the victims loose on them. That would be justice.