Whether killing animals is comparable to murder is a different question than whether animals are people.
Peter Singer argues against the species-ist argument that they're incomparable. Or that killing any human is worse than killing any (or even all) animals, i.e. he argues that for some human,animal pairs killing the human is less-worse than killing the animal.
http://www.princeton.edu/~psinger/faq.html :
> Q. I’ve read that you think humans and animals are equal. Do you really believe that a human being is no more valuable than an animal?
> A. I argued in the opening chapter of Animal Liberation that humans and animals are equal in the sense that the fact that a being is human does not mean that we should give the interests of that being preference over the similar interests of other beings. That would be speciesism, and wrong for the same reasons that racism and sexism are wrong. Pain is equally bad, if it is felt by a human being or a mouse. We should treat beings as individuals, rather than as members of a species. But that doesn’t mean that all individuals are equally valuable – see my answer to the next question for more details.