Believing in consciousness without free will is epistemically unjustifiable. This is the universally accepted theory of animals, but since it's necessarily irrational, there are two possibilities:
Animals don't have consciousness.
Animals have free will and rights.
I can't know which is true. They don't seem to act like people, but they also seem too close to be naturally occurring robots. If you sincerely think the probability of #2 is so low that it's okay to eat meat, then I don't blame you, and I think a reasonable person can think that. But for me the probability is about 50/50, so for me, killing an animal is half as bad as murder.
There are other complexities, and I'm not covering them all here. I will link Michael Huemer's piece.