What I have seen first hand as a difference between a trained dog that will "guard" your family and an untrained dog that will protect a family is... once the switch is flipped there is no off switch.
Where a well trained police dog or otherwise may quit on command. I have seen a dog act out of protection of their owner and would stop at nothing.
So, when you saw the uncontrolled attack, was the person being attacked killed? If you can't stop a dog that is attacking to protect someone, then the person will typically die or at least take life threatening injuries because the dog's intent is to KILL them......IF it really is protective instinct.
