A friend of mine from grade school and high school was staying at his parents' house a few years ago when his father woke him up--his mother had stopped breathing. Well, Greg started to do CPR, and actually broke a few of his mother's ribs while performing it. It didn't revive her. 
I'd guess a significant part was the time between when the patient stopped breathing and when CPR was begun. The number of minutes you get are single-digit. and past 5 minutes permanent damage sets in.
Even if you witness the patient go down, activate the EMS and begin CPR with AED assistance the odds of survival are scant.
... I think I'd rather be sued or fired than to do that.
I have an acquaintance who had a gun pulled on him. He got the better of the would-be mugger and walked away.
But he was also an EMT.
They called it "abandonment."
He lost his EMT, did time as a felon, lost the right to own a gun, lost his secuity clearance.
They destroyed him. He now works as a maintenance technician.
There is a broad chasm between the law and justice and the law is very eager to prove itself.