As for 'being filled with smoke", it goes both ways. If the perp can see to shoot, then trust that you can see to shoot back.
As for 'gas", while the mask helps him to breath in a plain smokey room, there is no confirmation that it was "gas". I have watched the videos, and no one is coming out of the theater as if gassed. All comments were "harder to breath", which would be caused by a regular smoke grenade. No one says eyes stinging. Were it gas, you'd have seen it just with the folks at the front of the theater. I have been gassed a dozen times. Had my nose run off my chin.
His guns were legal purchase. Signal smokes are easily purchased. But CS type of grenades ... no f'ing way.
Even so, someone near him, with a handgun, who escaped the initial shots, and likely beside him, or now behind him, can certainly do damage. There were many places to aim that would have disabled him and taken him down. Likely from less than 10' range.
Your posts pretend to speak with knowledge on this subject. But I see no knowledge. I see uninformed speculation, to the point of ignorance.