A Long Island nightclub turned into a house of horrors yesterday when two men were shot — one fatally — in a massive fight over who should have won a Halloween costume contest, cops said.

The $1,000 dress-up competition at the Dream nightclub in Hampton Bays turned ugly at about 4 a.m., after a dispute broke out over which of the final two contestants — a woman dressed as Lady Gaga and another dressed as a cavewoman — had won, police said.

Police said that 64-year-old Carlo Petrusa of Riverhead was shot and killed in the brawl.