>if you barred the doors to a theater and tossed a few gallons of bleach and vinegar in you could also just gas hundreds of people, but it doesn't happen.

Well during the 2012 Aurora movie theater shooting the guy used homemade tear gas before killing 12 and shooting another 60. But you're right he didn't lock the doors. The 2016 Pulse shooter did lock the doors and killed 50 and shot an additional 50. So it's not as far fetched as you make it sound.