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If it's just the bank being robbed, even though part of me wants to play "hero," I don't know that it would be wise to intervene, for several reasons. One, if I'm laying on the ground with my hands above my head (as it seems the robber ordered everyone to do), I'm at a tactical disadvantage. Two, since it's the bank being robbed (not me personally) I might be at a legal disadvantage. Three, since there are likely many bystanders at many different angles to me and the robber, there is a greater risk of a firefight leading to their injury, so I am at an ethical disadvantage.

Now, if the robber turns to all of us customers and demand we hand over wallets, etc., then that changes things. Also, if he not only threatens force but actually uses it, that would extremely alter the rules and more than anything else would probably compel me to action.