What is the scene at the end of "American Gangster" after the credits?

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For those of you who saw American Gangster and waited til after the credits, there's a 5 second scene where Frank Lucas is in a bar or diner. He pulls out a gun and shoots the camera. You can also see a man to the right recoil from the gun shot (he might have been the one getting shot or was surprised by the sound). What is the purpose of that scene? Frank, during that scene, has already gotten out of jail. Who does he kill?