By way of historical investigation, I think you might have better luck if you approached your topic differently. While historians often have a vague idea of what an answer might look like before they start investigating, it's usually better to go into research with an open mind rather than a set hypothesis.

If I were you, I'd start reading around books on the history of morality, or rather the history of ethics (which would fall under intellectual history, history of philosophy, history of thought categories). Dive into the arguments that different scholars make; chase down their own secondary sources but especially their primary ones.

Then, see what kind of argument emerges to you out of those sources. :)