Who can? Apparently not everyone has the right to do this. You rationalistic types trumpet loudly that "man doesn't need G-d to tell him right from wrong" and then when a fellow human exercises his evolution-given "right" to invent a moral code you start screaming bloody murder, as though there were some "objective" moral standard that exists outside human invention.
You decide to live by "live and let live," someone else decides to obliterate whole populations . . . what's the dif? Both of you are doing what "seems right to you." Does the obliterator need to check with you as to what is right and wrong? Can't he decide for himself? Or do you think that people like you have some special "right" to decide what is right and wrong for everyone else because your "rationalism" gives you a "clearer view of reality?"
C'mon . . . you don't believe people have souls! What does it matter if someone discriminates or litters or wipes out an entire country? There can't be any metaphysical consequences. After all, it's not like people have "souls" or anything, is it?
Ever since your prececessor Jefferson you people have been claiming that your own subjective moral hang-ups are "self-evidently" true.