For the past 11 summers, the village of Sagaponack, on the eastern end of Long Island, has been the site of a Monday-night ritual hippie-ishly incongruous with the moneyed community of homeowners in this part of the Hamptons. At Sagg Main Beach, a flat, broad ribbon of sand dividing the Atlantic Ocean from Sagaponack Pond, the crowds begin to gather near six P.M., when a small circle of musicians starts pounding out Brazilian music on drums and sleigh bells, and beachgoers cluster around to listen and dance and take in the sunset with a beer.

Last September 1, at a drum circle near the end of the season, 32-year-old Peter Smith was hanging out with family and friends when he saw a face at once familiar and unwelcome. Smith’s family had been coming to Sagaponack since 1973; his father, a retired Lazard Frères banker, had bought a historic house just down the road from the beach. The house had a timber frame and gabled roof, and part of it dated to the 1600s; Mr. Smith (also named Peter) was house-proud, often seen in a painter’s cap and tool belt as he devoted countless hours to the home’s meticulous restoration. But it was no museum piece, instead serving as a hub for loved ones and the larger community, including friends like Louis Malle, Larry Rivers, and former roommate Oliver Stone, and for years it was the site of an annual blowout summer party that drew as many as 800 people.

Gilbert at his arraignment in State Supreme Court in Manhattan on February 5, 2015. By Steven Hirsch.

Peter the younger, a co-founder and C.O.O. of a shop-local e-commerce start-up called MadeClose, was popular and sociable, but when Tommy Gilbert approached him on the beach that evening, he was terrified, friends of Smith’s say. (Smith declined to be interviewed.) Only two years earlier, the young men had been friends, with much in common. Both had attended the elite Buckley boys’ school, on Manhattan’s Upper East Side. Both came from families who belonged to the insular Wasp preserve known as the Maidstone Club, in East Hampton. Both loved to surf. For a time, they had been roommates in Brooklyn.

But then Gilbert had developed an obsessive vendetta against Smith, according to mutual friends, which culminated in a brutal attack on the street in October of 2013 that put Smith in the hospital and led him to obtain an order of protection against Gilbert. Now, though, nearly a year later, as vaguely tribal rhythms competed with the crashing surf for the crowd’s attention, Gilbert, in violation of the order of protection, “literally came out of the darkness,” someone close to Smith says, and ominously told Smith it was his last chance to have the order rescinded and be his friend once more. As others physically blocked Gilbert from Smith, “he was trying to get Peter to leave the crowd and walk down the dark, desolate beach with him, away from the bonfires and people.”

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Four months later, the Smith house on Sagg Main Street would be in ashes, Gilbert’s father dead with a bullet wound in his head, Tommy in jail, and large swaths of New York society horrified and bewildered.

“A Little Bit Off”

On paper, Tommy Gilbert seemed sprung from the daydreams of F. Scott Fitzgerald. He grew up in what his father called “a small mansion” in Tuxedo Park, New York, and then at various tony Manhattan addresses (a Park Avenue apartment, a town house on East 61st Street). He was the son of an investment banker and a former debutante and was raised in an environment of extreme privilege. From Buckley, he’d gone to Deerfield Academy, then Princeton. He was a strapping six feet three inches, gym-bodied and catalogue-model-handsome, with a man-bun-worthy sweep of blond hair and piercing blue eyes. Anyone glancing at the galleries of black-tie society pictures online would have seen a dashing man-about-town, seemingly at ease among his fellow swells.