Jury nullification
A person who overreacts when seeing someone commit a crime may be guilty of a crime themselves. For example, if a child is killed by a drunk driver and the father then kills the drunk driver, according to law the father may be guilty of murder. The same is true if a man seeing someone sexually assaulting his wife or daughter and beats the attacker to death (in many jurisdictions). A jury is made up of people who live in a community. They can decide to let someone go free if they think is it safe to do so.