You seem to be obfuscating the act of killing with the euphemism of "eating" to make your question more palatable.
Which is worse, killing dogs or having sex with dogs? You have to choose one.
You're thinking of this taboo completely wrong. Consent is an anachronism when most marriage in the past was arranged or bartered.
The reason there's a taboo against beastiality is the same as miscegenation: disease.
Sex with animals and sex with foreigners both spread disease.
DISCLAIMER: I am not passing moral judgment on beastiality/miscegenation, I am merely pointing out the subconscious reasons why humans have opposed both.
What diseases did Europeans get from the Colombian exchange? Syphilis. What did Aboriginal Americans get? Everything else.