John Locke in his Second Treatise of Government had two premises for natural moral law: independence and equality. Independence means that we think and feel individually. Equality is about moral worth. There is no inherent master/slave status in human nature. There is no inherent superiority or inferiority among the races, sexes, or other categories of human beings. The moral default is therefore equality.
This is the concept recognized by Thomas Jefferson, when he wrote that all persons are created equal. This is the concept of equality before the law. Human equality is a premise for natural moral law, or the universal ethic.
When one person imposes coercive harm on another, he makes himself master, and the other is a slave. This is inconsistent with equality. The universal ethic begins with our subjective values, and then provides a moral production function resulting in moral rules for the universal ethic. One’s personal ethic or subjective value that being coercively harmed is evil gets passed as a universal ethic moral rule that coercive harm to others is evil. But mere offense becomes transformed as morally neutral, since one has not been invaded.
For the full treatment, see my book The Soul of Liberty.