My wife and son and I have discussed this scenario quite a few times.
If we or they are in an enclosed public place, mall, restaurant, or whatever:
1) Identify location of nearest exit at all times.
2) If seated, sit near an emergency exit or the kitchen (back door access)
3) When in the mall or other large shopping complex pay attention to the sights and sounds around you at all times. Odds are if you hear yelling or see someone running, there's a problem.
4) If we, they or I am/are threatened with imminent harm, do not hesitate to eliminate the threat by whatever means necessary.
We've also talked about pulling fire alarms if they are accessible.