The book was thought-provoking and I enjoy every moment of it.
The idea of good comes from the noble class. It is no surprise that noble means good in dictionaries. Bad which is opposite of good, therefore must represents non-noble or common class.
Society enforces morality on its members through punishments. We learn fast through pain. The stronger the society, the more lenient it is to the violators.
Guilt comes from creditors and debtors. Being indebted to someone will result in being guilty.
We are indebted to our ancestors, we feel guilty that the debt can never be repaid. The stronger the tribe, the bigger the debt feels like. As the tribe grows more powerful, ancestors become bigger and scarier. In the end, the later transformed into heroes and gods.
With the rise of atheism and the decline of religions, the mankind's feeling of guilt subsides.
I'm putting reading the book on hold; I may come back in the next few years.