Serial nice guy MICHAEL C. HALL, who plays TV’s scary serial killer “Dexter,” just learned that with fans, no good deed goes unpunished!

While dining with a pal at chi-chi Chin Chin’s on Sunset, a male fan approached, asked for an autograph, and Hall – who usually asks people to wait until he’s leaving – dropped his chopsticks and cheerfully Hancock’d!

But minutes later, the guy’s whole family ringed his table, babbling like a banshee clan!!

“As nicely as he could, Michael said they were being intrusive, and that he’d appreciate being left alone,” said My SpyWitness.

“So the jerk stormed off, muttering nastily, ‘I can see why his marriage (to “Dexter” co-star JENNIFER CARPENTER) broke up!’ Hall took the snide comment in stride, but he’s getting more protective of his privacy,” said the source.

“Even when he left the restaurant, people were yelling ‘DEXTER’ at him from their cars. That unnerved him. He knows people have dark sides – he’s playing a serial killer. And who knows when some crazed fan might try to off him!”