The stunned man, clutching his wound, begins to stagger around in the street as the suspect continues menacing him with the knife, as if threatening to strike again.
The victim finally collapses in front of a green cab before the killer walks away from him.
Police said the victim later died at Coney Island Hospital. His identity has not yet been released.
The killer was captured at the scene. Charges against him are pending.
He told cops the victim had tried to rob him and he pulled the knife to defend himself, officials said.
The incident is still under investigation.
Local resident Angel Reyes, 26, said he saw the video when he entered the store as workers were reviewing the tape.