i've never heard of a carnivore who thought meat eating was morally better.

Katja Grace claimed to me that being a total utilitarian led her to prefer eating meat, since eating animals creates a reason for the animals to exist in the first place, and she imagines they'd prefer to exist for a while, and then be slaughtered, than not exist at all.

I tend to hang out in the average utilitarian camp, so that one didn't move me much. On the other hand:

Oh, you want utilitarian logic? One serving of utilitarian logic coming up: Even in the unlikely chance that some moron did manage to confer sentience on chickens, it's your research that stands the best chance of discovering the fact and doing something about it. If you can complete your work even slightly faster by not messing around with your diet, then, counterintuitive as it may seem, the best thing you can do to save the greatest number of possibly-sentient who-knows-whats is not wasting time on wild guesses about what might be intelligent. It's not like the house elves haven't prepared the food already, regardless of what you take onto your plate.

Harry considered this for a moment. It was a rather seductive line of reasoning -

Good! said Slytherin. I'm glad you see now that the most moral thing to do is to sacrifice the lives of sentient beings for your own convenience, to feed your dreadful appetites, for the sick pleasure of ripping them apart with your teeth -

What? Harry thought indignantly. Which side are you on here?

His inner Slytherin's mental voice was grim. You too will someday embrace the doctrine... that the end justifies the meats. This was followed by some mental snickering.

I'm pretty sure that the maximally healthy diet for me contains meat, that I can be maximally effective in my chosen goals when maximally healthy, and that my likely moral impact on the world makes sacrifices on the order of a cow per year (note that cows are big and hamburgers are small) look like a rounding error.