Humanity should respect animals.

[Tenet Nosce]

I don't really see how one could respect a cow then kill it unnecessarily.

Exactly. Which is why I would question if "respect" is the best principle to apply to the situation.

I think I understand your point, which is why have any sort of boundaries at all?

I am saying that there should be a boundary between ethics, and other philosophical considerations. Ethical philosophy is essentially about human-human relationships. We can attempt to apply ethical principles outside of these, but it is problematic. I'm not saying it is a totally wrong approach. I'm just saying it is problematic. And the evidence of this would be the fact that despite 6000 years of ethical arguments for various dietary practices, we still haven't arrived at a general consensus.

Philosophically, it can reduced to some simple guidelines: When faced with a choice between compassion and cruelty, choose compassion. When faced with a choice between a violent way of solving a problem and a non-violent way, choose the non-violent way. Do what we can to prevent being in a situation where we have to destroy life in self-defense, but if we must do that, then do so consciously and realize that there might be some karma involved. Do what we can to minimize it and compensate if possible. We can't completely avoid harming other lifeforms, but we should do our part to minimize the harm.

These guidelines are ethical guidelines that are based upon the concept of harm. And I wouldn't necessarily disagree with them. What I am discussing is whether ethics is the most appropriate branch of philosophy to apply to the situation.