The adrenaline is pumping hard. People are panicking and running everywhere. The gunman is standing in an area surrounded by innocent people. If you take a shot you run the very great risk of hitting an innocent person.
There are no general answers beyond trying to mentally rehearse a potential bad situation and be comfortable with the weapon you do have.
All else is conjecture or bravado — or both.
Thank you for your reply. That is exactly what I was trying to do, mentally rehearse many scenarios of what and when to do something. The “pat” answers by macho would be heroes in their minds, are annoying.