Mr. Garner’s pleas before his death — “I can’t breathe,” repeated 11 times — helped to galvanize the Black Lives Matter movement, which itself led to a counteroffensive from the police, who believed the criticism interfered with their ability to do their jobs.

Commissioner O’Neill said he strove to make a decision “unaffected by public opinions demanding one outcome over another.” Anticipating blowback from all sides, he got personal, similar to what he’s done in the recent past, whether in eulogies for fallen officers or in his pointed apology for the police conduct that led to the Stonewall Riots.

He spoke of his visceral reaction to the video depicting the struggle between Mr. Garner and Officer Pantaleo, even after watching it countless times.

“Every time I watched the video, I say to myself, as probably all of you do, to Mr. Garner: ‘Don’t do it. Comply,’” Commissioner O’Neill said. “To Officer Pantaleo: ‘Don’t do it.’ I said that about the decisions made by both Officer Pantaleo and Mr. Garner.”

It was a speech steeped in context, one that sought to turn the discussion from hashtags on social media back to the original scene. Commissioner O’Neill described in detail the undesirable conditions near that corner of Staten Island the day Mr. Garner died.

“Neighborhood residents purposely avoided the area in and directly around Tompkinsville Park,” he said. “Drug dealers worked the edges of the park, and across the street, selling narcotics. A handful of men regularly sold loose cigarettes made cheaper by the fact that New York State taxes had not been paid on them. A liquor store nearby sold alcohol to people who would drink that alcohol in the park — people who would sometimes use drugs, urinate and pass out on benches there.”