Good advice - I heard a police presenter on active shooters who said that if an armed citizen starts moving around, he or she probably will get shot.
An LEO here told me that he knows of a guy who was running to a mall where there was a shooting with his gun out to save his wife. He came within a hair's breath of being shot.
How can LEOs know who is a good guy? Some rampage shooters from the professional classes come to the scene dressed professionally and open up. Some can be professionals who work there. Look at Amy Bishop - if the police arrive and she didn't give up, the campus cops might know her as a professor. Is she an armed good person professor or a killer?