I've read this book, and the one on demonic possession after it, and they're actually really awkward and uncomfortable partly because they don't just blame it on mental illness.

Peck mentions (so briefly, wtf!) that he was a Colonel in the US military and part of the inquiry into the My Lai massacre, so part of his discussion is what could lead those soldiers to make the decisions they did.

IDK, I'm not sure I recommend these books exactly? They're so WTFy and all over the place and full of weird. But they're also soooo much more complicated than the easy answer.