Why do people turn to cannibalism? There's desperation for food, and there are the psychotic murders, and… well, in other times and other places, cannibalism has been used for other purposes, not all of them nefarious.
Non-survival cannibalism is a whole other thing. And it didn’t used to be that uncommon either. Cultures all over the world have incorporated human flesh into rituals and events. Some of these rituals, like eating the flesh of a recently deceased person at the funeral, have positive associations. Some, meant to intimidate enemies, involved eating the flesh of their warriors. It’s not necessary to go that far back in the past to find that sort of intimidation, either. In World War II, a few Japanese soldiers were tried with war crimes for cannibalism. Except the U.S. realized it hadn’t really ever technically outlawed cannibalism in international law so it had to technically try them for something else.