What to do in an active shooter situation?

I’ve always seen videos in which cops have engaged in shootouts with criminals and thugs etc. Anyways I’ve always wondered, if you are present during a situation is it a good idea to pull your own gun and assist the cops or simply get out? Basically, how does a cop know in a shootout who a bad guy is and who the good guy is? via /r/CCW https://ift.tt/2xDq5Mc