I think the victim’s family should decide how to kill the convict.
Slippery slope - leading off in all sorts of nasty directions.
That's why punishment for crime should be a community decision. (neither the decision of an oligarchy nor a vendictive induvidual) If you don't like the way your community punishes crime, move. If the oligarchy is the problem, deal with that oligarchy!
e.g. Don't do drugs in Singapore! Or chew gum or 'key' parked cars.