We could have a law that applies to the penalty with respect to outlining undenialble proof, eye witnesses, DNA evidence,
past etc attached to the penalty phase. The parameters would be set and met prior to penalty phase with the need for 2 eyewitnesses taking priority, or steep factual evidence before preceding. Of course all of this would delineated and should be well defined with no leniency in the definition of the parameters. Thats the best we can do before we hand it over to the switch puller, i.e. the family. There are ways. This isn't funny but here goes a riddle: A woman attends a funeral of her mother and at the funeral she meets a nice young man and speaks with him for a while. They hit it off. He leaves then she remembers that forgot to get his telephone #. After the funeral she immediately goes out and kills her sister why?
Here is the answer, She figured she could see him again at the sisters funeral. Now here is the scary part, this question was posed to inmates at one prison with every serial killer getting is right.