What if I told you cannibalism wasn't really all that awful? What if I told you that cannibalism was socially optimal? What if I told you that banning cannibalism could harm localized social welfare?
You might look at me funny, but I think there may be situations under which cannibalism is the only sensible strategy under certain institutional and productive constraints. Bear with me.
The highlands of Papua New Guinea are pretty well known for infertile land, mineral-poor volcanic rock substrate, thick jungle, craggy landscapes and little sustainable animal husbandry. Obtaining food energy is expensive: the highlands staple of Sago is hard to harvest and process, not particularly easy to store and extremely labor-intensive. Banana, sweet potato and taro crops are relatively recent innovations and tend to do poorly in the grotty landscape. Quite naturally, you might imagine that in lieu of producing, there could be one heck of an incentive to become a raider. With that in mind, it's similarly wise to provide a little extra incentive to dissuade raids.
Killing a hard-to-detect interloper just might not be enough. Since the decision to raid is likely to be one made by the entire tribe, reducing the status of the raiding tribe would be in the interests of the defenders.
So what to do then? Suppose I'm in Tribe S and J.R. is in Tribe J (clever naming convention, huh?) I'm a raider type and I send off my best thief-in-the-night to bag some bananas from Tribe J. They catch my fellow and have to decide what to do with him. They could hold him hostage and trade him for some of my resources, they could torture and kill him, they could just bash his head in and toss him off the side of a cliff, they could take him as slave labor or they could kill him and eat him. Of all of the death-centered punishments, cannibalism seems the wisest decision. New Guinea highlanders live a Malthusian existence poor in protein and wasting effort detaining or torturing interlopers expends valuable calories. In addition to the nutritive value of the raider they eat, they also get to send a powerful signal to Tribe S: raid us and your men will end up dead in the service of making us stronger. It's both instrumental and expressive punishment. Note also that under these desperate Malthusian conditions, ransom may not be adaptive all the time--Tribe S may simply not have enough resources to ransom a captive. Bummer.
So here we are, chugging along with our little SPNE working out more or less okay and along comes the Great and Terrible (Explorer) Mungowitz with his little armada. He makes his way up into the hills and is simply appalled that people are eating other people. He therefore righteously and with all the vigor of God begins smiting the filthy heathens. Justified that the Word of God has been brought to the benighted savages (me and Jeff), Mungowitz, head held aloft, wades triumphantly through the surf back to his dinghy and thence to the gentle shores of Nag's Head, mission well and good accomplished.
But what of us left behind? In the absence of some alternative institutions, in the absence of replacement lawful order, Munger's feel-good intervention just up and plunged our little island into terrible chaos. We now lack raid prevention institutions and given a pervasive lack of social trust, shift from already marginal production to more and more theft, further discouraging production. Pretty soon we all starve and die. Thanks for all the humanitarianism, buddy.
So, here we have a nominally revolting arrangement shattered in the name of best intentions to a yet more dire end. Wise and circumspect interventions simply must understand why it is institutions exist before embarking on reform campaigns. The safe default assumption must be conservative in the sense that non-adaptive institutions just simply tend to not last. Also, if you want to reform, a better way to do it is to offer superior alternatives rather than simply banning existing practices or removing alternatives. You don't like sweatshops? Great, start a factory of your own and pay what you think is a fair wage. Don't petition state-sponsored goons to shut down people's livelihoods with axes and water cannons. You don't think coffee supply chains benefit farmers? Okay, find out exactly how and why the supply chains work. Feel-good "fair" trade beans might not help them any more. Don't presume that just because you see exchange that is ostensibly non-euvoluntary that you can help people through coercion.
And most importantly, don't go making assumptions. You make an ass out of u and umption.