Why was this 23 year old boy shot to death in cold blood by officers?

He was shot because the 23 year old MAN was obviously threatening either an officers or other person's life with the firearm. Police were called there because of domestic violence. They arrive and he's wielding a gun. He doesn't put the weapon down. A judgement call is made at that point. Maybe he made a sudden movement or pointed the gun and the officer had to react.