Generally speaking the criminal does check out any other patrons, and if they see something they don't like go elsewhere. This happened the first time with me when a group of people came in at exactly the right time and the scum-bag went to the store next door instead. The second time I was looking straight into into the guys eyes when he was trying to make up his mind (BTW I'm 6'5" and and athletic 220, a nice guy, but at first glance I do not fit the category of "victim" ). He decided to try elsewhere also and went back to his buddy's car (which they had backed into a parking spot at the far end of the building, my first clue they were up to no good, could they have been MORE obvious. Amusingly enough that prevented them from seeing the cop car that had parked on the other side of the building to do paperwork). I do not know the circumstances that occurred with my parents as they have never told me the details, and my personal sample is way to small to make absolute conclusions from. However, there have been studies, and they support what I mentioned before.

Of course none of that matters with the truly drugged out or crazy. If one of them come in your odds of death (or worse) go way up and there aint much you can do to prevent them from trying, they just don't care. The only chance you have is either being able to physically overpower them or someone else being there who can, but someone is getting hurt, probably badly.