In the Grant Houses, a public-housing project in West Harlem, he was known simply as Bam, short for the superstrong cartoon baby Bamm-Bamm in “The Flintstones.” He had a reputation as a fierce street fighter. At 15, he posted a video in which he knocks a man out cold in a hallway.

Prosecutors say Taylonn Murphy was also eager to be a leader in the housing project’s gang, a loosely aligned group of older teenagers who called themselves 3 Staccs and regularly battled two nearby crews, Money Avenue and the Make It Happen Boys — sometimes with fists or pipes, sometimes with guns.

Those feuds, played out on the streets and on social media, eventually led to the murder of Mr. Murphy’s sister, Tayshana, in a dingy high-rise hallway in September 2011. Ms. Murphy was a high school basketball star, scouted by several colleges, and her fatal shooting became emblematic of the toll of crew violence over turf and respect.

Nearly three years after the killing, the police raided the Grant Houses and neighboring Manhattanville Houses, arresting 103 young men on conspiracy charges in one of the largest roundups of people suspected of gang membership in New York City’s history, a sweep intended to dismantle the crews and end the violence.