As a medic, I am a first responder. However my job does not begin until the scene is secure.
My work is a gun free zone. Our response is to evacuate if in immediate area. If not in immedicate area, then lockdown your section by locking doors or barricading, turning off the lights. Fight only if in immediate danger. It's how we train.
If off duty and I am at a public place, I will secure my immediate area, aid others to escape, and if in immediate area of shooter I will do my best to neutralize the shooter.
Seriously, a lot of things could go wrong, however it's not going to stop me from trying to do what I can.