One of the most tear-your-hair-out depressing moments in the Darfur documentary “The Devil Came on Horseback” comes when the filmmakers visit Rwanda, the site of another modern genocide. Their camera tours a morgue where skeletons from Rwanda’s recent ethnic cleansing are preserved in lime and laid out — reminders that the West’s promise of “Never Again” has, at least when it comes to Africa, proved an empty one.

What makes the footage especially disturbing is that it follows a steady barrage of images from Darfur: charred human remains, blood-spattered earth, villages going up in flames. The bone-white human remains on display in Rwanda represent that country’s unspeakable tragedy, but they also seem to herald a similar fate for the Darfur region of nearby Sudan.

More than 200,000 have died in Darfur’s four-and-a-half-year-old conflict, and 2.5 million have been forced from their homes, but the government-aided decimation of Sudan’s African tribes is not yet history. Brian Steidle, the steely protagonist of Annie Sundberg and Ricki Stern’s fitfully engaging documentary, is convinced that something that can still be done to stop it.

Mr. Steidle is not your ordinary activist, and this is the primary difference between “The Devil Came on Horseback” and so many other hand-wringing Africa treatises. He’s a former U.S. Marine captain from a family of military men. After four years of service, he turned down a desk job to become a ceasefire observer for the African Union in Sudan — in which capacity he witnessed horrors unlike anything he’d ever seen.

As roving militias called the Janjaweed — Arabic for “devils on horseback” — sweep virtually unimpeded through western Sudan, shooting, raping, and hacking to death unarmed villagers, he uses camera and notepad to document the atrocities. Mr. Steidle leaves shaken by the experience, and angry: Why was nothing being done? In interviews, he recalls, Janjaweed chieftains freely admitted their alliance with Sudan’s Arab government. Why did Mr. Steidle’s superiors insist on keeping his work secret? “If these photos were released to the public, there would be troops here in a matter of days,” he says.

Mr. Steidle later discovers the naiveté of that notion. Upon returning to America, he shares the photographs with anyone who will look, and while they get plenty of media attention — and make him a VIP at a “Save Darfur” rally in Washington, D.C. — it doesn’t put army boots on the ground, and the killing in Darfur continues.

This self-effacing ex-soldier remains devoted to the cause, however; he is clearly a results-driven man, and willing to put his dreams of early retirement (and passion for sailing) on hold. Mr. Steidle, who declares that it is both right and necessary to “use force to protect people” and claims he often wished he’d been allowed to carry a gun in Darfur, is also not your typical human-rights crusader. He has no problem calling the Janjaweed evil. It’s refreshing to see a guy like this pleading Africa’s case.

That said, Ms. Sundberg and Ms. Stern’s spotlight on him is misplaced. This is yet another a film about Africa with a white protagonist, but the issue here isn’t that Mr. Steidle is white — it’s that he’s not a natural protagonist. Other observers of Darfur, even if they didn’t see what he saw, certainly could have offered insight. And Mr. Steidle, despite — or perhaps because of — his integrity and discipline, lacks the on-screen personality to carry an 85-minute film. E-mails he wrote his sister from the field, which he reads in voice-over, offer assessments such as, “The general situation here is tense.”

This is probably why the film opens with enough jump cuts and electric guitar to make you think you’re watching “Black Hawk Down.” But the filmmakers would have done better to take their cues from the most moving scene, in which Mr. Steidle interviews an elegant, English-speaking Sudanese man in a refugee camp. As the man explains that America has done much to help him and his family, while his fellow Muslims have done nothing, he seems overcome with both gratitude and distress. After shaking Mr. Steidle’s hand, he wanders behind a tent, where he stands, uncertain and unaware that he is being observed, his back to the camera. It’s a brief but direct encounter with the reality of Darfur, and it speaks volumes.