Yeah it must of been the circumstance ,like Costello may have been already walking in the apartment so Chin could not get off another shot , possibly a bodyguard could of been in area or others .

Because very strange Chin did not unload the gun knowing he may not live blowing the hit .... wonder if it was stage fright and Chin wanted to run .... just weird knowing he missed.

Then there could be possibly that Chin thought he was dead and would not fire another shot because that would be my reflex reaction to make sure.