I'm all for challenging preconceptions, but if "human" is an arbitrary definition for the sake of attaching an ethical imperative to it, then in dispensing with it you can't simply choose a different but equally arbitrary definition. And if you do, you should stick with it. Most people probably would be upset with killing a puppy but wouldn't call it "murder", but if a moral vegetarian does, then it's no problem if they also call it murder to kill a cow. So where's the border they use? Generally, they would also object to killing a
Obviously, any ethical imperative will seem silly if you do what I just did to it, take it to ridiculous extremes. That's no argument against having them. But the point I'm angling at is that every moral vegetarian has as arbitrary a border to what constitutes murder as every non-vegetarian. And, to extend the argument the other direction, cannibals could level the same accusation at non-cannibals. In a way cannibals have the most internally consistent, non-arbitrary version of the rule: if you can eat it, you may eat it. (No, I'm not advocating cannibalism here.)
But you don't have to take the argument to extremes to put a fine point on the arbitrariness of moral vegetarianism. For as much as they talk about "taking a life" they can't avoid the fact that that carrot was also alive, and if equating murder with humanity is "speciesist" then equating life with animals is "kingdomist". Things are no less alive because they don't walk, any more than because they don't talk or file income taxes.
Some address this point by only eating bits that come off of living plants: if you pluck an ear of corn and eat it, the plant lives on. Or even go so far as to only eat things that fell on their own. Yet those same people often object to eating eggs; and if an egg is a potential chicken, so too is an apple a potential apple tree in precisely the same way. All that luscious fruit flesh around the seeds is just as necessary for the seeds to survive and thrive, as is all that tasty protein in the egg, for the chicken to live. In fact, since the egg's unfertilized but the seeds are ready to grow, eating an apple is arguably more grievous than eating an egg.
Which brings me to the really juicy bit of this argument. Moral vegetarians are very keen on the idea of getting by without having to kill anything for their food. That's the highest moral standard for them: they strive always to do as little killing as possible to sustain themselves. So what do they kill and eat? Autotrophs. The only creatures on the planet that don't kill anything for their food. The only creatures on the planet that actually meet the moral vegetarian's code of morality perfectly and unreservedly... the moral vegetarians kill and eat them.