#1 TAKE COVER
#2 Assess the situation
#3 Decide whether you want to HIDE, FLEE, or FIGHT
#4 Reassess the situation
#5 Evaluate possible courses of action to accomplish #3
#6 Reassess the situation
#7 Take course of action.
Watching one of the videos from the Paris attack on a shop, there were some excellent opportunities for an armed citizen to take out the gunman.
In most active shooter situations (if not all) the gunmen are standing in the open not even trying to use cover. An armed citizen effectively using cover will not be mistaken for an active shooter - especially if he disengages and remains behind cover when the police arrive. Now if he's an idiot and stands out in the open firing at the active shooter, or waving his gun around after the police arrive, then yeah, he could easily get shot by the police.
