If there's a cop in the food court, and a guy starts shooting up the food court, I would assume the cop is going to shoot the guy. Why would I pull a gun?
If I'm in the food court, and some guy starts shooting up the food court, and there's no cop around, why wouldn't I try to shoot the guy? I shouldn't pull my gun, because the cops might accidentally shoot me? There's a guy that is going to shoot me on purpose- that's my priority.
If you're saying don't pull your gun out at the Chick-Fil-A at the highway, run across the mall parking lot, through Macy's, the length of the mall, to the food court, yeah, that's probably a bad idea.
I just think the fear is way overblown. When I think of spree killings in public venues, I can think of maybe a dozen cases in which armed people have intervened. I can think of two who were shot. Neither of them were shot by police, and one of them never pulled his gun, because he was afraid of being shot by police.
You should be cognizant of it. You shouldn't be handling a gun after the killer is no longer a threat. You should be ready to drop a gun if the cops show up. You should accept that there is some risk of getting shot by a cop anyway, but if you're already engaging a killer you've assumed some risk.
Heck, you face some risk just being there unarmed. A few years back Bloomberg sent out a story about a mass shooting in NYC where 10 people were shot, then quickly took it down, because 8 of them were bystanders hit by the police. Some guy shot his business partner and pulled his gun when police confronted him in a crowded area.