I would say human nature because that is the way people are inclined, you could not change that. The reason why the law has come up with capital punishment is because the crime committed by the person is deserving of death.
The restriction placed on just how severe a crime can be, murdering for fun without consideration that you have harmed another with unbearable pain is enough, Deuteronomy 19:19-21. The hearts of people to be able to see blackness in their own hearts. We can trust that God Almighty will be an accreditable judge who will be fair and accurate. There are some people that do not to care that other are incredibly violent and out of control, that is why the city breeds fear, Jeremiah 17:9-10. God has a powerful need to make changes instead of forcing people to live with increasing numbers of black "sheep". The prophesy of destruction fits the judgment call merited out to unrepentant sinners, 2 Thessalonians 1:6-10.