Brutal, urgent, devastating — the documentary “The Devil Came on Horseback” demands to be seen as soon as possible and by as many viewers as possible. An up-close, acutely painful call to action, the movie pivots on a young American, a former Marine captain named Brian Steidle, who for six months beginning in the fall of 2004 worked for the African Union as an unarmed monitor in Darfur. What he saw in Darfur was unspeakable. And then he returned home, his arms, heart and head filled with the images of the dead.

Image Mr. Steidle in Darfur. Credit... International Film Circuit

You see a lot of those images in “The Devil Came on Horseback,” which, in brute form, serves as a catalog of human barbarism. The title refers to the Arab militias known as the janjaweed, which, sponsored by Sudan’s Arab government, have been instrumental in waging a campaign of violence and terror against the inhabitants of Darfur, many of them black. At least 200,000 civilians have died, and millions have been displaced. The atrocities — rape, torture, mutilation, murder — seem endless. So too does Mr. Steidle’s storehouse of graphic photographs and his documentation, which he took with him when he returned to the United States and began sharing with anyone who would pay attention, including Nicholas D. Kristof, a columnist for The New York Times, who wrote about him in a March 2, 2005, column titled “The American Witness.”

Directed by Annie Sundberg and Ricki Stern, “The Devil Came on Horseback” is a heartfelt account of what this particular American witness saw and, just as important, what he did afterward. It’s necessary, often agonizing viewing, but it’s also something of a frustrating mess, marred by overly flashy, obtrusive editing and sloppy use of unattributed news sources. Mr. Steidle is an intensely empathetic figure, but sunny images of him from his own childhood threaten to take the documentary perilously off course. His humanity and humility are inspiring, transparent; not once do you feel as if he’s playing to the camera to show just how bad he feels. He doesn’t sell his pain, so there’s no reason for the documentary to either.