I don't think it's ethically okay to kill and eat animals, so my opinions on this may be biased. However, to take a stab at this, I suspect that most people live in the illusion that animals are perfectly happy with the lives they have on big industry farms and so on and that they have a sudden, oblivious, painless and "humane" death when they die. Thus, while their lives are cut short, their lives aren't rife with opportunity anyway so they're not missing out on much, and they don't actually suffer at all. Supposedly. I guess.

It's not okay to kill and eat humans because that gives rise to all sorts of diseases in the mad cow family, among others. I suspect that the taboo against canniblism comes from learning thousands of years ago that sometimes when you eat other people, you and possibly everyone around you dies.

I don't know how communion comes into play.

And I'm not fully vegan yet, which puts me on the highly morally questionable ground of thinking it's not okay to kill animals but it's okay to keep them alive and torture them (i.e. use them for dairy the way our dairy industry works). I need to work on my resolve on that issue. Perhaps buying only free-range, organic dairy from farmers whose conditions I can visit and inspect would be better. I dunno. Or I could just get a backbone and give up the cheese.

On the other hand, my entire family back home has decided that they're going to eat nothing but veal from now on.