Out of the thinkers I know, Kant most rigorously theorized Ethics, tying it with Duty, Categories, Freedom, Universal Law, Causality (outside of nature), Immortality, and God. But his insight is remarkably simple: the very state of acting ethically is qualitatively different from everything else we do (what he calls “pathological actions”). It must be done without motivations for anything else, not as a means but as an end-in-itself. The good is the good despite everything against it.
It is nothing new, really. But can be seen already in Gorgias, that it is better to suffer injustice to do them. The act of doing something evil cannot be traded by anything else. It is in a realm in itself. It must be so, else it cannot be good. We know, for example, that love should be pursued at all costs.
This is an extremely demanding ethic. But ethics, perhaps, cannot be easy.