But what I am trying to get at is an underlying moral principle behind all this. Why is it OK to kill a creature for food as long as it does not suffer any pain in the process?
Would you be willing to accept somebody wanting to kill a dog for food?
So is the underlying moral principle that what is natural is good?
Yes, even as a dog owner, I would do what humans have always done in extremis; I'd eat dog meat. I see no difference between the validities of any living organisms and accept all as being equal. Just think, if there were no bacteria or fungi at the most basic levels of life, the rest would just not have evolved. So, the biosphere needs bacteria more than anything else.
I don't think that the idea of evolution should impose any straight-jacket of hierarchy or order of importance in which any one species or group of spp is 'higher' than another; we are plainly interdependent.
At this time in the history of the biosphere we seem to be realising too late that, for the continued well being of human popns, everything depends on everything else for survival and if we destroy and outbreed everything else on the planet , then we reduce the likelihood of our own successful continuance. We need to find a way of reducing human popns to planetary sustainable levels and to stop treating the world as a giant chemistry set.
This sounds a bit or-else apocalyptic, I know, but it might form the basis for the underlying 'natural' morality you seem to crave.
I don't think it will matter one jot though: we're not smart enough to think our way out of it; 6.6 billion and still rising - Soylent Green here we come!
Sorry this post rambles on a bit, I'm nearly ready for my bo bos, but I hope you can see what I mean.