Why does good people sin?

"Everything is good in nature." according to philosophy, the mere fact that it exist proves that it is good. Man exist and therefore is good in themselves. But why does sin? it is because man learn to be selfish as they grow by the support of his genes, culture and experiences. Once a person forget to think about others just to think for his self may cause the person to hurt anyone for the profit of once self. No matter what sin it is: Abortion, Corruption, Terrorism, War, Racism, etc... it boils on one thing it is Narcissism. A selfish cause of death. Man is the great enemy of himself, People sin because of the people and we blame it to poverty, to anger, to hatred, to crisis, to Government, to politicians and to anything that can be blame. but the reality is we can not accept in ourselves that our selfishness cause everything when one person commit a little selfishness -thinking it is not that big so that is okay.- Just imagine when million of people in different country have this kind of mentality then it would make a big disgrace. So, why does good people sin? don't ask me why, ask your selves.