Then put handcuffs on him and arrest him and take him to jail and indict him.
You don't beat him to death.
The kid wasn't the danger on the street -- he was harmless.
The cop who killed him was/is the one with the really dangerous mental illness. He was a bomb ready to explode and odds are that he has more incidents like this in his past.
Uncle chip, with all due respect, you are uninformed.