I doubt whether it is possible to answer your original question meaningfully while excluding all reference to religion. Until very recent times, virtually everybody who thought seriously (or even not-do-seriously) about human nature did so within some sort of religious framework, and, even now, most people who think about the matter probably do so against a background of some sort of religious belief. Their views about human nature will not really make sense divorced from that context.
That’s a fair point, but it’s one thing for a philosophy to sprout from someone and have it mesh with his religion. Aquinas on numerous subjects comes to mind. It’s another to have the theory be part of the religion. But for what I am looking for, a religious component makes it unhelpful. That’s why I thought moving into to psychiatry/physiology might be helpful. For instance, is their a component of dopamine release when we help people, etc.
Rousseau is the only philosopher I’m aware of who definitely stated that people are inherently good and only become evil when they are corrupted by society. How inherently good humans came to create wicked, corrupt societies, he does not say. In any case, Rousseau was a pretty corrupt character himself.
It sounds like you are interested in the social science aspect, i.e. the economics of altruism.
This may be a little daunting, but there are 2 Handbooks on the subject, if you have access to a university library. Don’t be reluctant to skip over the indecipherable greek letters.
Handbook of the Economics of Giving, Altruism and Reciprocity, Volume 1: Foundations (Handbooks in Economics) Amazon.com
Handbook of the Economics of Giving, Altruism and Reciprocity, Volume 2: Applications (Handbooks in Economics) Amazon.com
There may be something there. Or there may be nothing there. But the Handbook series tends to provide a very good roundup of economic research, and those editions were published last decade.
Robert Wright may or may not have something to say. I enjoyed The Moral Animal but have not read this one: Amazon.com
You might also google for Altruism in Experiments, Prepared for the New Palgrave Dictionary of Economics, 2007 by James Andreoni, William T. Harbaugh, and Lise Vesterlund. Read the abstact and the first couple of paragraphs. Skip to the conclusion. Then circle back or try pulling the references if you feel like it.