One good blow to the head with a club/bat/bar could knock someone out cold. It wouldn't be an unreasonable reaction to draw a weapon.
I would draw and give one verbal warning. If its dark, I can't see what he is holding. All I know is that he is disgruntled for some reason and charging at me with a long object that could very well be a rifle or shotgun...Even if it was a bat it could still be deadly. If he didn't immediately stop after my verbal warning and seeing a gun pointed at him (Anyone with any sort of brains would turn around and run at this point), I would open fire.