By C. J. CHIVERS,

The New York Time

The Syrian rebels posed casually, standing over their prisoners with firearms pointed down at the shirtless and terrified men.

The prisoners, seven in all, were captured Syrian soldiers. Five were trussed, their backs marked with red welts. They kept their faces pressed to the dirt as the rebels’ commander recited a bitter revolutionary verse.

“For fifty years, they are companions to corruption,” he said. “We swear to the Lord of the Throne, that this is our oath: We will take revenge.”

The moment the poem ended, the commander, known as “the Uncle,” fired a bullet into the back of the first prisoner’s head. His gunmen followed suit, promptly killing all the men at their feet.

This scene, documented in a video smuggled out of Syria a few days ago by a former rebel who grew disgusted by the killings, offers a dark insight into how many rebels have adopted some of the same brutal and ruthless tactics as the regime they are trying to overthrow.

As the United States debates whether to support the Obama administration’s proposal that Syrian forces should be attacked for using chemical weapons against civilians, this video, shot in April, joins a growing body of evidence of an increasingly criminal environment populated by gangs of highwaymen, kidnappers and killers.

The video also offers a reminder of the foreign policy puzzle the United States faces in finding rebel allies as some members of Congress, including Senator John McCain, press for more robust military support for the opposition.

In the more than two years this civil war has carried on, a large part of the Syrian opposition has formed a loose command structure that has found support from several Arab nations, and, to a more limited degree, the West. Other elements of the opposition have assumed an extremist cast, and openly allied with Al Qaeda.

Across much of Syria, where rebels with Western support live and fight, areas outside of government influence have evolved into a complex guerrilla and criminal landscape.

That has raised the prospect that American military action could inadvertently strengthen Islamic extremists and criminals.

Abdul Samad Issa, 37, the rebel commander leading his fighters through the executions of the captured soldiers, illustrates that very risk.

Known in northern Syria as “the Uncle” because two of his deputies are his nephews, Mr. Issa leads a relatively unknown group of fewer than 300 fighters, one of his former aides said. The former aide, who smuggled the video out of Syria, is not being identified for security reasons.

A trader and livestock herder before the war, Mr. Issa formed a fighting group early in the uprising by using his own money to buy weapons and underwrite the fighters’ expenses.

His motivation, his former aide said, was just as the poem he recited said: revenge.

In Washington on Wednesday, Secretary of State John Kerry addressed the issue of radicalized rebels in an exchange with Representative Michael McCaul, a Texas Republican. Mr. Kerry insisted, “There is a real moderate opposition that exists.”

Mr. Kerry said that there were 70,000 to 100,000 “oppositionists.” Of these, he said, some 15 percent to 20 percent were “bad guys” or extremists.

Mr. McCaul responded by saying he had been told in briefings that half of the opposition fighters were extremists.

Much of the concern among American officials has focused on two groups that acknowledge ties to Al Qaeda. These groups — the Nusra Front and the Islamic State in Iraq and Syria — have attracted foreign jihadis, used terrorist tactics and vowed to create a society in Syria ruled by their severe interpretation of Islamic law.

They have established a firm presence in parts of Aleppo and Idlib Provinces and in the northern provincial capital of Raqqa and in Deir al-Zour, to the east on the Iraqi border.

Karam Shoumali contributed reporting from Antakya, Turkey; Anne Barnard from Beirut, Lebanon; and Michael R. Gordon from Washington.