"Active shooter" is definitely a stupid term, for three reasons.
First, in many cases the psycho does not use a gun. Was the guy who went around stabbing people in the Minnesota mall an "active shooter with a knife"?
Second, I can't think of what would qualify as a "passive shooter." A guy sitting in his recliner with a beer in one hand, and a pistol in the other hand, deciding to just wave the thing around and not shoot? "Shoot" is an active verb. If somebody is engaging in the act of shooting, then "shooter" is an accurate description; "active" can be assumed.
Third, "active shooter" -- or more correctly "shooter" -- does not describe intent or justification. The off-duty cop who shot the Minnesota mall stabber was a "shooter," and he was definitely active about it. But we don't classify him as an "active shooter."
The term annoys me like Hillary Clinton's voice, but I am resigned to just going with it, because it has become part of the cultural language. Everybody pretty much gets the idea.