The standard is already beyond a shadow of a doubt and requires the prosecution, original judge, jury, several rounds of appeals overseen by more judges. After which the governor or president can weigh in.

Humans are fallible and therefore incapable of creating an infallible justice system.

Allowing any death penalty means murdering some number of innocent people to "get" a greater number of bad people who you already have locked up.

So answer the question. How many innocent people are sufficiently low collateral damage and stop pretending we can only execute bad people.