A security staffer who got dragged into a bar fight he was trying to break up just proves the old adage is true ... no good deed goes unpunished, and sometimes you gotta kick ass.

Check out this wild footage, obtained by TMZ, showing an absolute melee going down late Friday night/early Saturday morning at a joint called Shorts Bar & Grill out in Fairport, NY ... where we're told 4 dudes went at it right on the floor as the evening tipped into midnight.

As exciting as that was ... it was nothing compared to when one of the hired "Staff" security guards stepped in. At first, he was pulling people away and separating everyone -- that is, until someone on the outskirts apparently pushed him down to keep the chaos going.

That's certainly what it looks like, anyway ... and that's certainly how the bouncer took it -- 'cause the guy started wailing on the man whom he suspected of shoving/provoking.

Indeed ... the beatdown he dished out to this older fella is pretty brutal -- dropping elbows on him and socking away. Eventually, they let the guy get up and stumble his way out of the scene. He didn't really put up much of a fight, and it's unclear what happened after.

No word on whether cops were called to this brawl or not either ... and what's even weirder is that this seems to have been Buffalo Bills related (if the jersey'd fan is any indication).