Most of these shooters aren't tactically trained. I don't know what the odds are of someone accurately shooting a moving target is with regards to stopping them immediately, but it's probably fairly low. Have you ever seen the video of a guy shooting his lawyer outside the courthouse at an arm's length distance? He emptied the whole gun and the guy lived.
I'll take my chances even if the odds are poor. When you take no action, the Twin Towers crumble to the ground. When you take action, the bad guys plans get ruined (
UA flight 93)
One time I ran with the intention of tackling a man with an axe. It was 3am and he was hacking at a car. A moment before tackling I recognized him. He had accidentally locked his keys in the trunk of his car and thought opening the trunk with an axe was a good idea. I probably should have called the police instead, but he didn't see me and I had the element of surprise on my side.
There's a reason my signature says what it says.