If you are seeking morality deriving from this plane of existence, you will never be able to define "good" and "evil." From a purely rational perspective, morality is not real; that is the reason for secularists using the term "ethics." Ethics is just a series of rules agreed upon by members of society, no different from legal statutes; neither has any authority or legitimacy other than via application of force.
In the modern Western cesspool that has excised God from their society, the only authority and legitimacy derives from force and power. The strong does what he can, and the weak suffer what he must. That is reality and truth.
That is a pretty bleak view of it. I think that even a person alone who has never experienced a society would still create a set of rules with which to govern their life. Thy wouldn't require a god either.
The set of rules created ex nihilo has no value, legitimacy, or authority than paranoid delusions of a schizophrenic. Just because a collection of men document their wish list does not make such a list imbued with universal values. Morality cannot exist without invoking the divine.