Actually, while there are a few cases in which cannibalism is seen as something okay, in most cultures, it's seen as horrifying. When the Africans were taken as chattel slaves to the Americas way back when, they thought the Whites would eat them, for instance, whereas the Whites thought that the Africans would eat them, had they the chance. So, in the materialistic sense, cannibalism has always been uncouth, at least in anthropological records. (I think this information is from Complicity, though it may be from somewhere else... maybe The Story of American Freedom?... sorry for the lack of specifics!)
The instances where cannibalism is seen as something positive are when the spirits of the people are brought into those who are consuming the person. So, for example, a modern example of this kind of mentality belongs to certain Christian faiths (Orthodox and Catholic, primarily) that believe they are eating Jesus Christ when they eat communion, and by bringing in His Spirit, they become more fully one in Christ. This is not quite cannibalism, because those types of Christians believe they are eating Christ, who is God, so it's different, but in true cannibalism, eating the flesh of another human, when viewed in a positive light is almost entirely because of the spiritual connotations rather than the material ones.
Still, because of that spiritual view, eating one's flesh can be seen as negative. For example, there are some cultures that forbid eating meat because they believe that people are reincarnated into animals, and it's just something not to be done. To possess the human spirit in that way through consumption is seen as a kind of evil, or an abuse of power.
So yeah. Different ways of looking at it! However, I think that the main reason why eating humans is so horrifying for us is because of these spiritual implications, even if you may not be devout or religious at all.

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