In Illinois we have what is called "the good samaritain law" If you find someone in trouble Weather it be from getting mugged to someone having a heart attack. You as the aide giver cannot be punished or sued by the attacker or in the case of preforming CPR the person you were trying to save. Obviously in the case of seeing someone commit a burglary or something of that nature you do not attempt to stop them, you call 911 but in the event that that someone is being brutally harmed, you may step in and defend the victim using as much force that is necessary to insure that the victin is safe. If you are preforming CPR on a victim then you may do whatever is necessary to insure the body has blood flow and oxygen to the brain. So if you break ribs in the process you cannot be sued.