They came for the aging Bloods soldier a few minutes before midnight, bursting from the lobby of a housing project with a loaded 9-millimeter handgun.

The first bullet probably killed him, but six more shots followed anyway. And then the assailants scurried off into the shadows. Six brass shells lay in a halo at Jequan Lawrence’s feet. Another rested beside his head: a point-blank shot at an already-dead man that would leave his face disfigured at an open-casket funeral.

In a South Bronx neighborhood where violence is hardly rare, the brutality of the ambush gave even the most hardened residents and detectives pause.

But it meant even more to Mr. Lawrence’s fellow Bloods. It disabused them of the notion, however naïve, that their gang’s red flag of loyalty prevented one Blood from killing another over a petty beef.