I doubt that most people would take the option to do it. Despite what you might think, most people do not have it in them to torture/murder someone, even if that person did something horrible to them. They might be able to do it if they are heavily effected by their grief, maybe, but once that subsides and they come to terms with what they actually did... it would just be a lot emotionally and mentally for them to take. That's why they don't have sign up sheets outside of prisons letting people sign up to be the executioner; killing someone takes a lot of mental strength and preparation that the majority of people do not have.