Besides, even aside from ethics, killing other healthy survivors for a very short-term profit (like some gun rounds) or no reason at all seems like a decidedly stupid move in the zombie apocalypse. You would assume that in a world with an omni-present common foe, which you can't negotiate with, and which can creep up to you at any time, you would want to maximize the number of friendly people around you (if for no other reason than that they can keep watch while you are sleeping). Yet, we generally see Rick et al. try to minimize the size of their group if at all possible. I see that food might be an issue, but given enough people, there really shouldn't be a problem just growing your food yourself.