Just to clarify: would you say that was morally, ethically, judicially and religiously wrong for God to allow Cain, Moses, David the King, and Saul of Tarsus to keep their lives after having committed murder?
Yes, because only the murder victim or, of course, God can forgive the murderer if he admits his crime and asks for forgiveness. As for man, murder (the deliberate taking of an innocent’s life) is the only sin that God commands the sinner shall be put to death, and this requirement appears in each of the first five books of the Bible.