And do you really think a darkened room full of amateur gun owners opening return fire is going to in any way lessen the death toll? Against a gunman with body armour?
It probably would, actually. Even the best armor doesn't make you immune to bullets - penetration or not, all that energy has to go somewhere, and when the bullet hits an armor plate, it's basically translated to a very heavy and rapid punch of the plate against the body. This results in, at the very least, a massive hematoma, and quite possibly in broken ribs, depending on what exactly the round was.
Alternatively, if this is soft armor (e.g. kevlar alone - what police typically use unless it's SWAT), then the bullet actually creates a bump on the other side that can easily be 2-3 inches deep - and if there's body behind the plate, then that's what gets protruded by said bump. There are safety standards that define the maximum size of such bumps, but their point is to make sure the person wearing the armor survives, not that they're not damaged at all.
So yes, several people unloading handguns at the armored shooter at the same time would, at the very least, knock him down and hurt him significantly, possibly enough to buy more time for others to get out of the way, and possibly even to disable and subdue him.
FWIW, the Aurora shooter wasn't actually wearing body armor. He had a plate carrier that was capable of accepting armor plates, but he didn't actually have them in it, using it simply as a load bearing vest. He did have armor on some other parts of the body (head, neck, legs), so it's probably because he was buying things without understanding what they are, and bought a plate carrier thinking that it is armor.