In the course of interacting with other individuals, the philosophy of ethics develops. As individuals, we each have our own morality. If we did not interact, this morality would be sufficient justification for any action that the individual might take. The meeting and interaction of different moralities is what motivates ethics. It is contrary to the study of ethics, therefore, to base any ethical system solely on the self.
Any ethical system motivated by self-interest alone is no different than individual morality.
A society that lived by this ethic would gain no benefits from having an ethical system at all. It is, in essence, a choice to have no ethical system at all. For ethical systems concerned with a mix between personal freedom and societal good, the areas of personal freedom are still areas where morals reign, and conflict will still occur in those areas. The social nature of ethics brings about a certain level of necessary Utilitarianism to any ethical system. Ethics becomes a language for the communication of right and wrong, but it must be communicated in a manner that is consistent with its purpose.