An ethical system like that of John Q. Murderer is inconsistent with these basic assumptions. To take Kant's categorical imperitive again, it might be that someone who thinks the earth would be better off without humans could pick up a weapon and kill everyone within reach, and consistently will that everyone else do the same. Of course, some people may not. I seem to recall a discussion with you in which I made such an argument, and you claimed not to see anything ethically wrong with such behavior. So, if you don't see anything inherently unethical about killing and eating a five year old, there's not much left to argue.
Arbitrariness is also a concrete concept.
If we're going to allow arbitrary distinctions, it would be just as consistent for me to claim that only whites, or only males, or only white males with brown eyes and brown hair and computer science degrees from the University of Maryland, deserve ethical consideration. To be satisfactory and meaningful, such claims have to be supported by appeal to some relavent property of the group under discussion. It's said that 'power corrupts', but actually it's more true that power attracts the corruptible. The sane are usually attracted by other things than power. When they do act, they think of it as service, which has limits.