Imagine somebody you love being dead - and imagine someone else treating their flesh with disrespect, or even "desecrating" it (or eating it). My personal reaction is an immediate, visceral rage. I'm pretty sure I would attack that person almost instantly.
I think that's because I view their flesh as
belonging to them - in the sense of ownership - so while the mind and soul I loved may be gone, I would not tolerate disrespect or hostility shown towards what once was their most intimate belongings. Like I would not tolerate disrespect shown to belongings that my loved ones once held dear - even more so, in fact. Disrespect shown to their flesh is disrespect shown to the person I loved.
While not everybody might share that view, most functional human beings have enough "theory of mind" to predict my hostile reaction - and hence they would be very cautious.
There's lots of speculation around the taboo against human cannibalism - there are some very good medical reasons against consuming our own dead, the most dangerous one being that it's an excellent way to
contract prion-related diseases similar to Alzheimers. Members of your own species have the same immune-system vulnerabilities that you do - whatever nasty shit they have swimming around in their bodies is highly likely to be dangerous to you, too - whereas the number of diseases that are able to jump species are limited.