A NEW documentary, “The Act of Killing,” explores the human capacity for mass murder. It addresses the Indonesian fratricide of the mid-1960s, in which a million people may have been killed.

The slaughter was monstrous, but it was also mystifying — which is the way it often is. I’ve interviewed war criminals in a half-dozen countries, and it’s always bewildering how the nice old person across from me, so graciously concerned with how much milk I want in my coffee, could have committed atrocities.

The puzzle of such episodes is that otherwise good and decent people were so oblivious to the abhorrence of what was going on. So I was struck that the same section of this newspaper that carried a thoughtful review by A. O. Scott of “Act of Killing” also reviewed another documentary. That one was “Blackfish,” and it looks at the SeaWorld marine park and its (mis)treatment of orcas.

Orcas, also known as killer whales, are sophisticated mammals whose brains may be more complex than our own. They belong in the open sea and seem to suffer severe physical and mental distress when forced to live in tanks. Maybe that is why they sometimes go berserk and attack trainers. You or I might also go nuts if we were forced to live our lives locked up in a closet to entertain orcas.