As Jordan Neely struggled to free himself from a chokehold in the New York City subway earlier this month, there were the passengers who pinned him down and the passengers who watched.

Two men helped restrain Mr. Neely while Daniel Penny, an ex-Marine, held him on the floor of an F train that had stopped in a Manhattan station, a four-minute video of the May 1 episode shows.

Around 10 passengers observed the three holding down Mr. Neely, 30, who slipped into unconsciousness. A woman tried to walk around the cluster of people on the floor, but seeing Mr. Neely flail his legs, she bit her lip and stepped back, the video shows. Another woman typed on her phone, looked at Mr. Neely then glanced out the subway doors. One man stepped into the train and told Mr. Penny, “You’re going to kill him.” He was not seen to physically intervene.

Nor, under New York law, did he have to.

Nationally, the killing of Mr. Neely, who the police said had been acting in a “hostile and erratic manner,” has set off a broad political debate about vigilante behavior: Many Democrats called for Mr. Penny’s arrest, while conservative political figures called him a hero. Supporters have donated almost $2.5 million for his defense.

But in New York, one of the nation’s most densely populated cities, the death has prompted conversation about what moral duty onlookers have to one another and to the most troubled people among them.

The legal standard is clear: No U.S. state explicitly requires civilian strangers to physically intervene when they see an adult in danger, though some impose a duty to report wrongdoing and two set an ambiguous standard of rendering assistance. The value of such legislation has been debated for years, according to people who study the intersection of ethics, the law and bystanderism — the phenomenon of being less likely to intervene when there are others present.

In New York, where residents rub shoulders in corridors, crowded sidewalks and packed subway cars, the decision of how to respond to an uncomfortable situation is a daily dilemma.

There is an unspoken code in the city: “You do not get involved, you cannot solve every person’s problems. We learn to be very guarded,” said Ken Levy, a professor at the Paul M. Hebert Law Center at Louisiana State University, who studies bystanderism and who lived in New York for years.

“The closest we can come to undoing a tragedy like this is by blaming the people who did it, and those who didn’t stop it,” he added.

Only one person has been officially blamed. On Friday, Mr. Penny, 24, was charged with second-degree manslaughter. He has not yet been indicted or entered a plea. The men who helped hold Mr. Neely down have not been arrested or charged. It is unclear if the police know their identities.

The police interviewed at least several bystanders after the incident, according to internal documents obtained by The New York Times. The police and the Manhattan district attorney have since put out public calls for more witnesses to come forward.

Some New York activists believe the passengers share some culpability. During a wave of heated protests near and inside the Broadway-Lafayette Street subway station where the F train was sitting when Mr. Penny choked Mr. Neely, some demonstrators focused on those who merely watched.

Arrest “everyone that was on that train,” read one protester’s sign. “Love and protect your fellow man. Don’t move here if you’re scared of your neighbors,” read another. It added: “If you watch someone choke and attack/kill another, you are complicit. You are responsible.”

Debate over if or when bystanders should intervene intensified after the 1964 sexual assault and murder of Kitty Genovese in Queens. The New York Times initially reported that 38 people had witnessed the attack at various points but did nothing to help. (The Times has since acknowledged that the report “grossly exaggerated the number of witnesses and what they had perceived.”)

The ubiquity of cameras has intensified the question. In 2019, at least 50 teenagers witnessed the murder of 16-year-old Khaseen Morris, who was stabbed outside a strip mall in Long Island. Some filmed his killing on their phones but did nothing to help, a detective said at the time.

In a case like Mr. Neely’s, where an individual’s behavior may be frightening, authorities say bystanders should tread lightly. On Tuesday, Mayor Eric Adams said New Yorkers should call the authorities “if a person is doing something extremely dangerous so proper personnel could respond. And that includes mental health professionals — and, in those cases where it’s needed, a law enforcement professional.”

Still, there is not always cellphone service in the subway. In the Neely case, at least two people alerted the train conductor, and the conductor notified the police, Juan Alberto Vazquez, a freelance journalist who filmed the four-minute video, said in an interview the day after the killing.

Some states have tried to guide citizens’ actions in the heat of the moment, passing so-called duty to report laws.

Twenty-nine — not including New York — require bystanders to notify the authorities about a person in peril, or to assist so long as doing so would not endanger themselves or others, said Zachary D. Kaufman, a law professor at Boston University and the University of Houston who has compiled a database of such statutes worldwide.

Only two of those states have laws that may require onlookers to physically intervene. Rhode Island and Vermont require “reasonable assistance” when a bystander sees a person exposed to “grave physical harm,” Professor Kaufman added. Courts must interpret that language, he said.

Mr. Neely’s death also raised the question of who needed help, the victim or the passengers.

A few witnesses on the F train told authorities that Mr. Penny and the two other men who held Mr. Neely down were protecting them from what they perceived as frightening and unpredictable behavior, according to the police and Mr. Vazquez.

Mr. Neely screamed, “I don’t have food, I don’t have a drink, I’m fed up. I don’t mind going to jail and getting life in prison. I’m ready to die,’” Mr. Vazquez said.

“It was very tense situation, because you don’t know what he’s going to do afterwards,” he added. “None of us were thinking,” he said, that Mr. Neely would die from the chokehold.