One of the big questions in the debate over intervention in Syria has been: who are the rebels? Are they really moderates? Or doe the U.S. risk arming the kind of extremists who will turn against us in the long run? On Thursday, The New York Times' C.J. Chivers offers some unsettling evidence that the groups we think of as the good guys have engaged in "the same brutal and ruthless tactics as the regime they are trying to overthrow." In a video smuggled out of Syria by a disaffected rebel, a rebel commander called "the Uncle" stands over seven shirtless men, reads a poem declaring "We will take revenge," and then executes one prisoner. Gunmen then kill the rest.

In a congressional hearing on Wednesday, Secretary of State John Kerry was repeatedly asked about the ideology of the rebels. Kerry assured the lawmakers that only 15 percent to 25 percent of Syrian rebels are "bad guys" — extremists. "I just don't agree that a majority are al-Qaeda and the bad guys," Kerry said. He explained that the anti-Assad side "has increasingly become more defined by its moderation, more defined by the breadth of its membership, and more defined by its adherence to some, you know, democratic process and to an all-inclusive, minority-protecting constitution."

But Reuters' Mark Hosenball and Phil Stewart report that intelligence sources say the picture Kerry painted is optimistic. Yes, there are fewer extremists than moderates, but "Islamist groups like the al Qaeda-aligned Nusra Front are better organized, armed and trained." An intelligence source told Reuters that Kerry was right in saying a small minority of rebels were extremists. But another intelligence source said that in recent months, moderates might have lost ground, because they have fewer weapons and so have had to form alliances with extremists.

In late July, David Shedd, deputy director of the Defense Intelligence Agency, said publicly that there were 1,200 rebel groups, and "Left unchecked, I'm very concerned that the most radical elements will take over larger segments" of the rebel groups.

The Times report illustrates that potential. "The Uncle" in the execution video, 37-year-old Abdul Samad Issa, leads a group of fewer than 300 fighters. They have weapons because Issa bought them with his own money early in the civil war. Issa was a trader and livestock herder, the Times reports. Issa started as a protester, the Times reports, then started running a training camp, then this year started buying weapons, and by the spring, adopted the name Jund al Sham for his group. Three international terrorist groups use the same name. Issa promises his fighters "the extermination" of the Alawites, the sect Assad belongs to.

In June, Robert F. Worth's report for The New York Times Magazine showed the radicalization of some Syrian civilians. A young Sunni woman and a young Alawite woman had been best friends, until the rising sectarian fighting drove them apart. The Sunni woman now posts things on Facebook such as, "How many ‘likes’ for the conqueror of the Shia and other heathens?"

In the spring of 2011, Worth wrote, sectarian violence started creeping into anti-Assad protesters' rhetoric. 

One popular slogan was “We don’t want Iran, we don’t want Hezbollah, we want someone who fears God.” This may sound harmless to outsiders, but in Syria it was a clear call to Sunnis to rally against their enemies. During the summer of 2011, a bizarre rumor spread that if rebels banged on metal after midnight and uttered the right prayer during the holy month of Ramadan, Alawites would disappear. When I visited Aliaa’s home, she led me out to the balcony and showed me a terrace on the neighboring building. “You see that terrace?” she said. “They were banging on metal in the middle of the night. My father got out of bed and shouted: ‘Shut up! We’re not going to disappear!’ ”

(Photo via Associated Press: "In this citizen journalism Monday, Aug. 12, 2013, image provided by Aleppo Media Center AMC, which has been authenticated based on its contents and other AP reporting, Syrian rebels open fire as they battle against the Syrian forces in Aleppo, Syria.")