I think this is about the most important question there is and answering it isn't easy. There's a fundamental paradox involved. If people are born good, where did evil come from. There are religious answers to this and I'll leave those to the religious. I see this question purely in psychological terms. I'll just abreviate how, firstly to save my time, and secondly, because the nature of the answer is not something that people want to know anyway, so I'll just be wasting my time.
What would human consciousness be without language? There would be no knowledge about things, only the direct aperseption of reality. There would be only the eternal now, undevided, endless, as fully reflected by the human brain as possible. There would be no unconsciousness and no split, no self divided agoanst self.
All that is the result of language which makes comparison possible. This is good that is bad. I am good you are bad, you must be punished for being bad, taught molded controled, intimidated and threateded for this or that kind of act.
Again, since the capacity to feel the pain of blows and the sting of words is undefendable at first, the pain we felt was the maximum we could feel. To survive we split, we deny that we are guilty, do anything not to be the evil one. We learn to despise evil and in doing so learn to hate what we really are, the wrongly accused.
The reason that people are evil is that they hate their original self, the one who could have become God. Thus, while there is no evil, the feeling that we are evil makes it true.
Bottom line, we are born good only to be made to feel we are evil as quickly as possible. I guess the one good thing about the denial of these facts are that we aren't really evil anyway, but it would be nice to really know it.