Bashar Abdo had just returned home last month after four years in the Syrian army when a mob of neighbors and others armed with guns and knives stormed his family’s front door and accused him of being a thug for the ousted regime.

His sisters and sister-in-law tried to block the crowd while he hid. But people broke in and found Mr Abd, 22, in the kitchen. They stabbed him before dragging him out, although his sister Marwa clung to him. He was killed there.

The story shared by the family of Mr. Abda, confirmed the local police in the northwestern city of Idlib. A video widely shared on Syrian social media and verified by The New York Times captured the gruesome scene that followed: As Ms. Abdo held his lifeless body, neighbors continued to beat him. She begged them to stop, saying he was already dead.

“This is your destiny,” one man shouted. Other verified videos showed crowds chanting obscenities after Mr. Abd’s body was tied by the neck to a car and dragged through the streets. It is not clear who took the video.

Ms. Abdo recalled those moments in an interview with The Times four days later. She vowed revenge, a sign of the growing threat of a cycle of violent retaliation in the new Syria.

The country is suddenly and unexpectedly emerging from 13 years of civil war and more than five decades under the Assad dynasty, which held power through fear, torture and mass murder.

Mr. Abdo’s killing underscores the complicated showdown ahead in Syria, where wounds are fresh and anger close to the surface. Many Syrians claim responsibility for crimes committed during the civil war. Others seek revenge.

At least half a million Syrians have been killed during the war, most of them in airstrikes by Syrian warplanes and helicopters or in prisons under torture or mass executions, according to Syrian rights groups. Many people remain missing.

Officials in Syria’s new interim government, led by the Islamist rebel group Hayat Tahrir al-Sham, are rushing to establish courts and police forces to address decades of grievances. They call on citizens to forgive and not take matters into their own hands.

Ahmed al-Shara, the leader of the rebel alliance that toppled Assad’s government, said he would hunt and process high-ranking figures for crimes involving murder, wrongful imprisonment, torture and gassing of their own people, but that ordinary conscripts would receive amnesty.

IN recent interviewMr. al-Shara said that “justice must be sought through the judiciary and the law. Not through individuals.”

“If things stay the way everyone is taking revenge, we will turn into the law of the jungle,” he said.

Some Syrians said that while Mr. al-Shara might choose to forgive, they would not. Last week, the mayor of Dumar, a suburb of Damascus, was killed by residents who accused him of informing and arresting people under the former government, according to the Britain-based Syrian Observatory for Human Rights.

Mr. Abdo was a soldier — a conscript — in the Syrian army for four years. But his family said he tried to escape twice by failing to return after being given several days of leave. He ended up spending a month in a military prison for attempting to desert and was released when the rebels who toppled the Assad government seized the prison as part of their lightning rushes through the countryseveral family members said.

At first he was afraid to come home, but when he heard that Mr. al-Shara said that soldiers like him would be amnestied, he felt safe enough, his family said. Not long after he returned, the mob was at the front door.

They accused him of tipping off his neighbors, as a result of which they were killed or imprisoned. The family said that they see many murderers every day, but they did not confront them and are asking to be moved to another neighborhood.

Responding to questions about the killing, police in Idlib, which is affiliated with Hayat Tahrir al-Sham, which has ruled the province for years, said in a statement that it was investigating the killing, but that the Abdo family was “notorious for working with the regime.”

But police said “no one has the right to attack anyone”. No one has been arrested so far.

The family members denied that they had anything to do with the regime. They also said that if their brother had worked as a bailiff, he would not have returned home. They said he was just a pedestrian.

“We vowed that if the government does not get justice, we will get our justice,” screamed Ms. Abdo, 32, with tears streaming down her face. She slammed her fist into the carpet that she and her sisters had been washing for days to remove her brother’s blood. There was still blood in the kitchen and on some of the walls.

“We will not let his blood go unanswered,” she said.

Others use all means to try to avoid the cycle of retaliation.

Muhammad al-Asmar, a media official for the new government, said he sent a Google document to residents of his home village, Qabhani, in Hama province, to file any complaints against fellow villagers. Mr. al-Asmar said he took the initiative after hearing that several people the government relied on to abuse and intimidate Syrians had returned home after Mr. al-Assad.

“There was no response,” he said, because “people say: ‘I will take justice into my own hands’.”

Still, he hopes such an approach could be adopted nationally to stop vigilante justice.

Officials at the new justice ministry admit they were not ready to take over much of the country when they launched their offensive on November 27. For now, efforts to keep the peace seem to come in the form of public statements or suggested sermons for imams calling for people’s restraint.

“Honestly, we are under a lot of weight and there will be violations,” said Ahmad Hilal, the new chief judge at the Aleppo courthouse. People angered by the atrocities during the Assad era “don’t want to wait for the courts to act — they want to take law and justice into their own hands.”

The fight against mafia justice is terrifying because in every city and town, Syrians who could be accused of such crimes are returning home.

When Assad’s government fell last month, Alaa Khateeb returned to his village, Taftanaz, in a rural area of ​​Idlib province. His family quickly began telling people that he had avoided the military for years, then deserted twice to make it known that he was not a willing participant in the military Mr. al-Assad.

“I know I didn’t do anything,” said Mr. Khateeb, 25, a married father of three, on a recent day on the edge of the village, working to renovate a relative’s house that Syrian soldiers had seized and stripped bare.

Despite Mr. Khateeb’s protests, he faces a cloud of suspicion. Even lower-level conscripts are accused of enabling crimes – whether it’s true or not.

One of Mr. Khateeb’s relatives, Salah Khateeb, 67, who runs a market in the village, was not sure if he would even greet him when he heard that his other cousin had returned to Taftanaz.

“He is my cousin and I was wondering if I should accept him or not,” he said. “Others might even consider revenge.”

Muhammad Haj Kadour, Jacob Roubai and Nader Ibrahim contributed reporting.