Three men with long hair, wearing long beards and the robes favored by ultraconservatives, overheard the exchange. They accused the teenager of insulting the prophet, told him to leave the kiosk and then took him away in the car. When they returned an hour later, Muhammad bore the marks of a beating. In a square, they covered his head with his shirt, a makeshift blindfold, as if he were “some big shot,” his mother said.

She watched from her balcony, as hundreds of people gathered around. A resident of the neighborhood, Abu Abdo, heard what the men said. Addressing the “respected people of Aleppo,” they warned that cursing God or the Prophet Muhammad was a sin, saying it would be “punished this way.”

The teenager’s father heard the shots. His wife told him that the men had killed their son.

“His blood ran in front of me,” Muhammad’s mother said in a grim video posted on the Internet on Monday. In her anguish, she invoked the name of another teenage victim in Syria, Hamza Ali al-Khateeb, whose death at the hands of government forces helped propel the uprising, now a war in its third year. He was pulled aside two years ago by government agents at a protest, brutalized and killed before being sent back to his family.

Just as Hamza’s death crystallized the rage against President Bashar al-Assad, Muhammad’s killing stoked similar feelings against a new power that has emerged during the war. It focused anger on hard-line Islamists, including foreigners, some of whom have seized on the conflict in Syria as an opportunity to impose their mores. For Muhammad’s mother and some her neighbors, the tyrannies were indistinguishable, trapping many Syrians in a vise.

In a home in Muhammad’s neighborhood, Mr. Abu Abdo said what had happened to the two boys was “the same.”