Of all the deaths that have distinguished and propelled the Black Lives Matter movement, perhaps no other offers more powerful illustration of the dangerously broad latitude we give police officers to make claims of self-defense when they gun down civilians. As Mr. Small’s girlfriend, Zaquanna Albert, testified during the trial, Mr. Isaacs, an off-duty police officer in an unmarked car, had twice cut them off as they were traveling down Atlantic Avenue in the far left lane, after a long afternoon and evening of celebrating the July 4 holiday.

Driving with a baby in the car is a fraught enterprise under any circumstance, and Mr. Small was perhaps understandably undone by a maneuver that could have landed his family in an accident. He had a long history as a caretaker. His mother died of AIDS when she was 29, Mr. Small’s sister, Victoria Davis, told me during a break in the trial one afternoon, and he had tried to protect his younger siblings from a horrible foster-care situation. He had two older children with two other women, Sharon Seward and Monique Parker, both of whom remained close to Mr. Small and attended the trial proceedings. Mr. Small had been a longtime father figure to another child of Ms. Parker’s.

During the trial, when images of Mr. Small’s bloodied body were shown in the courtroom, the women in his life began to sob, and the judge ordered a brief recess. A video that had gone into circulation not long after Mr. Small was killed showed him approaching Mr. Isaac’s car from the driver’s side and then almost immediately getting shot and falling to the ground. Mr. Isaacs, who was represented by Stephen C. Worth, a lawyer who has made his name defending police officers who have killed civilians, said that Mr. Small had punched Mr. Isaacs through an open car window, something the video did not indicate.

But even if Mr. Small, who was unarmed, did throw a punch, it is hard to see how that could have possibly warranted grabbing a semiautomatic handgun and shooting him three times. If two men in a bar got into a fight over a Giants game and one punched the other causing him to draw a pistol and shoot his adversary, the shooter would struggle with a self-defense claim — the law in New York requires the use of parallel force. Lethal force can be justified in cases in which there is a reasonable perception of deadly or extremely violent threat — a punch in the jaw would presumably fail to pass the test.

Police officers do face a different standard when it comes to retreating from potentially dangerous situations, but the judge charged the jury to think about Mr. Isaacs as no different from any other driver.