When they met one night last June, Nicholas Brooks and Sylvie Cachay should not, their friends say, have been attracted to each other. Sylvie was an ambitious designer, once divorced, who sought out serious relationships with similarly driven men. Nick had recently dropped out of college, had never held a job for more than a few months, and had dated many women.

“They had nothing in common,” a friend of Sylvie’s says.

“Sylvie wasn’t typical for him,” a friend of Nick’s says.

But they did fall for each other, and by July, they could be seen around Sylvie’s West Village home, walking her miniature poodle or eating at the Little Owl. Nick, 24, and Sylvie, 33, were regulars at Employees Only and Cafe Cluny. They both dressed well and shopped at Sucre.

In the fall, the relationship turned volatile, and they repeatedly broke up and reconciled. On the evening of December 8, while in Sylvie’s apartment, Nick gave her some medication, possibly Xanax, according to a report he would later give police. She became groggy. Either he or she knocked a candle onto her bed, singeing her hair and lighting a small fire. At 12:30 a.m., to escape the smell, they left, then checked into Soho House, where Sylvie was a member. She reportedly told the desk clerk that she was “really tired” and incoherently complained about Nick. The desk clerk then helped her into the elevator and to Room 20, on the fourth floor. After Nick finished checking them in, he met Sylvie in the room. A staffer reported hearing Sylvie yelling at Nick...