The shooter did not confront police and surrendered immediately. People who shoot up bunches of unarmed people seem to stop right away in the face of any resist

The shooter did not confront police and surrendered immediately. People who shoot up bunches of unarmed people seem to stop right away in the face of any resistance. (As a counterpoint to me, you could point out that he was wearing body armor, as if he were expecting resistance. I'd say it was more like he was acting out some costume in his head.) An armed moviegoer wouldn't need to shoot the gunman to stop him. Even just firing in the air would have let the gunman know "someone else in here is armed and maybe shooting at me, I need to seek cover" and have significantly slowed him down. (Note that "firing in the air" is of course dangerous, since the bullets will hit stuff and stuff will fall on people. However, since we're talking about a gunman purposefully targeting unarmed people, that risk is much much less than the original risk being mitigated.)