In a fight or flight response your body doesn't just release adrenaline. It can also release other chems that inhibit your body from functioning properly (see the flight response in fight or flight).
Again, terrorist on a plane during 9/11, only one group of people fought back after the terrorist revealed the plane was bombed. The same thing in school shootings happens. Guy with limited ammo literally holds up hundreds of kids he couldn't possibly all kill, yet nobody makes a move to stop him. Take the columbine shooters, people in the library were WAITING to be executed by those mofo's, as they went from desk to desk killing people. They were even reloading while doing this.
And pausing and not running from some animals in the wild will spare you. Try to run away from a grizzly and he'll decapitate you as you flee. Sometimes it works, sometimes it doesn't.
(on the flipside if you pause or play dead with a brown bear, he'll eat you, scavenger).