Sources: FBI, Fox , 
quote: For example: A video shows John punches Bob in the face, he goes down nose bleeding. See, proof of assault has taken place, John is bad - must go to jail.But what about the 1-2 minutes before that. John walks in, minding his own business. Bob starts calling John names, pushing him, punching him. Laughing... he takes John's wallet. John gets up, punches Bob in the face, etc.