Then the answer is that eating sentient beings is not inherently evil in D&D. Eating your neighbor who you just murdered? Probably evil cannibalism. Eating a human prisoner who just wandered into Lizard Man territory for sustenance? Clearly not evil in D&D.
Indeed. There is no reason (except random happenstance) that the absolute morality system of a fantasy world should align perfectly with the ethics of the reader. I think it's especially true since I doubt the D&D community can agree on a common shared set of ethics, given it's worldwide diffusion.