When God wants to sap the enthusiasm of interventionists, he sends in the cannibals.

This is the best explanation I can come up with to help me understand the events in Syria over the past week or two.

I’ve been arguing (with intermittent hesitancy) that U.S. President Barack Obama is compelled to Do More in Syria to bring about the end of the regime of Bashar al-Assad. A lot of people have been arguing for him to Do More, most notably David Cameron, the British prime minister, who came to Washington early this week with that message.

Britain, like many of the U.S.’s perplexed allies, is waiting and hoping that Obama will take the decisive lead on the issue, because Cameron -- like the leaders of Turkey, Jordan, Israel and Saudi Arabia, to name just a few -- understands that only the U.S. has the heft to at least attempt to shape the outcome of the Syrian conflict to his liking.

The arguments for U.S. leadership are obvious: It is still the most powerful country on earth; our national security interests and our values just happen to align in the matter of Assad, in a way that they seldom do; and the regime in Syria is populated by torture-fiends and child-rapists.

But here’s the problem: The Sunni-dominated opposition to the regime has in its ranks a growing number of stone-cold killers, and, most consequentially -- at least from a public relations standpoint -- at least one cannibal.

The cannibal, Khalid al-Hamad, who goes by the nom de guerre Abu Sakkar, is affiliated with one of the several al-Qaeda-influenced groups in the opposition. He has become famous for a now-viral video in which he stands over the body of a dead pro-regime fighter and cuts into the corpse’s chest with a knife. He removes what appears to be a lung. “I swear to God we will eat your hearts and your livers, you soldiers of Bashar the dog,” he shouts. Then he bites into the lung. It is not known if he eventually found his way to the heart or liver.

When he was asked about his actions by a reporter from Time magazine, who interviewed him over Skype, Abu Sakkar explained why he was so upset with this particular dead government fighter: “We opened his cell phone, and I found a clip of a woman and her two daughters fully naked and he was humiliating them, and sticking a stick here and there.”

Government fighters, have, of course, committed acts so debasing and cruel to defy belief (read Human Rights Watch’s reporting on torture and rape as instruments of Assad policy), but now the opposition has in its ranks men who will commit wanton, debasing acts of violence with abandon. This is what happens when a civil war spins into chaos. What began as peaceful resistance became armed insurrection, and now armed insurrection is transmogrifying into mass slaughter, ethnic cleansing and the threat of genocide.

Abu Sakkar, in his interview, said of the members of Assad's Alawite sect, “Hopefully we will slaughter all of them.” He went on to brag that he used a saw to cut another pro-government fighter into pieces.

The bestiality exhibited by Abu Sakkar is not unknown in the region. Most infamously, one of the Palestinian assassins of the Jordanian Prime Minister Wasfi al-Tal, who was shot in the lobby of the Sheraton Hotel in Cairo in 1971, knelt by Tal’s body as he lay dying and lapped up the pooling blood with his tongue. The Middle East is a rough neighborhood. Still, the level of depravity exhibited by Abu Sakkar is rare.

I can only imagine what a U.S. president might think if confronted with this video. He could tell interventionists, with some justification: “I can’t believe you want me to go to the mat for an opposition that counts in its ranks men like this.”

To be sure, leaders of more moderate opposition groups have condemned Abu Sakkar. But they are not defining the contours of the revolt. Men who are responsible for another gruesome video posted this week -- men who run the al-Nusra Front, an al-Qaeda affiliate -- are the ones who, together with Assad’s forces, are pushing out the boundaries of cruelty.

In the video, a leader of the al-Nusra Front is seen making a statement before a long line of bound and blindfolded men, identified as government fighters. The men are on their knees and, the viewer assumes, know what is about to happen. The leader of the Nusra cell invokes Muslim Shariah law to justify their imminent execution, then takes out a pistol and puts it to the head of one of the kneeling men. The pistol seems to jam, and the Nusra leader turns, obviously frustrated, to fix the problem. He returns to the same kneeling man, and shoots him in the head. Then he shoots nearly a dozen other kneeling men, his fellow fighters calling out, “Allahu Akbar,” the whole while.

It is important to note that this sort of war crime was not perpetuated by rebel forces with any frequency during the first months of the uprising. But the actions of Assad's troops provoke reactions from the opposition, and now, here we are, with cannibalism, mutilation and mass executions.

Early intervention -- a coherent, active attempt by the U.S. and its allies to build up, finance and advise what was then a moderate opposition -- might have worked. Now, though, the Assad regime is showing signs of real resilience, and the opposition is showing signs of real brutality.

It is easy to blame Obama for his early passivity. It is slightly harder to blame him for looking at Syria as it is today and then choosing to ignore calls for deeper intervention on the side of the rebels.

(Jeffrey Goldberg is a Bloomberg View columnist. Follow him on Twitter.)