Millions of years of evolution has programmed within us a predisposition from birth to treat others the way we would want to be treated. That predisposition is then bolstered, enhanced, degraded, or left to atrophy like an unused muscle by familial and societal training and expectations, or by the lack thereof.
People are good or evil based on what they have learned and what they have taught. They are no such thing as good or evil by nature especially human. Human can be learned to be good or evil on what others have taught them.
We are born good, and learn to go bad from there. Humans are a social species, as such we have an instinct to be with others of our kind. We seek acceptance from the group which requires us to be respectful and caring to one another. This need for social cohesion is so strong that we have evolved the ability for empathy which allows us to "read" others to better help us fit in.
We are sinful by nature. That doesnt necessarily equate 'evil' as we think of evil. Think about it, tho... One of the very first, most natural things that an infant learns to do is to manipulate, decieve and 'lie' because it gets it what it wants faster very often. When a child begins to speak the word 'no' and lying are in its earliest vocabulary. Lying for gain comes far more naturally than learning not to lie, which has to specifically be taught.
People are good by nature and also fragile and limited by nature. Fragile and limited doesn't mean evil, but those things let you make or turn to evil. When we grow up we suffer more onrushes from environment (goods and evil), and the more we accept the goods from it, the more we become good; and the more we accept the evil, the more we become evil. I don't mean that being good or evil is the environment's fault. Of course it influences us, but it's all our choice, because we are beings able to make decisions, and our decisions will affect the environment. Hope that helps.
No one can adequately even define "good" or "evil" apart from the existence of moral absolutes. Coming from a Christian worldview, based on the Bible, I believe it teaches two things about mankind. 1. He and she are created in the 'image' of our Creator. We have a stamp of divine moral good in our conscience. Capable of love, kindness, etc? Of course. 2. Every person is 'fallen' in a moral sense, meaning that divine moral good is not eliminated but there is an innate moral tendency toward self-centeredness, a springboard of all evil. Jesus taught that from the 'heart' (no, not the organ) comes every kind of evil.
Certainly either can be influenced by people, experiences, teaching, etc. but the nature is there, even in a very young child, before environment has had a chance. Spend much time around even a 12- month old??