The last documentary that angered me (among MANY other emotions) was Dear Zachary: A Letter to a Son About His Father.
If you're softhearted, definitely get all the info on the case before you watch. If not, and you go in completely uninformed like I did, expect to be blindsided (perhaps a few times) and emotionally drained by the end of it.
To quote one review, "I found myself in the unusual position of feeling guilty for inflicting Dear Zachary on my wife, not because it wasn’t good, but because, in a way, it was too good and I was left without the one comfortable refrain I could usually rely upon in this situation: “It’s only a movie.” Sadly, it was also reality, a reality, perhaps, that you don’t want to get emotionally invested in."
If you'd like a short summary of the documentary topic, I'd be happy to give one. If you'd like a more detailed one with a "spoiler" on the turning point of the film, I'd be happy to oblige. All I'll say for now is this is likely the most powerful documentary I'll ever see.