Check out this video of a customer trying to choke out a bank robber. The robber got away.
Where did the customer go wrong?
(First, I should point out that the photo on this page of a bank robbery is not the same one as in the video.)
But James is right, in that the guy who tries to stop the robber makes two potentially deadly mistakes:
1) He does nothing to control the gun. As the bystander gets the robber into a choke, nothing is stopping the hoodlum from pulling the trigger, even if accidentally. An easy counter to the choke is to reach back and shoot the guy in the leg, foot, abdomen, etc.
2) He does nothing to control the robber’s base. As long as the robber can stand, and has the full use of his legs, he can try to shake off the choke, or even run out the door carrying his would-be subduer or stripping him off at some point.
I’ve seen other video of a guy choking a bank robber, but in that case the robber had no visible weapon (even though it’s always a good idea to assume that he has one hidden). Trying to choke a guy with a gun, without controlling the gun, and with as little training as this guy apparently has, is foolish and dangerous in the extreme. Only if he has reasons to believe that someone’s life is in immediate danger, let’s say the robber yells that he’s going to start shooting people unless he gets money, should he try to intervene.