Why are people selfish? Evolutionary psychology vs original sin
Evolutionary psychology: evolution made us act good, but we are not actually good
There's a big difference between someone acting good and someone being good. 'Acting' good is doing the right thing when there is some kind of selfish benefit to doing it. If people see me help someone out, then they will be more likely to respect me and I'll get a selfish benefit from it. Evolution can explain that very easily. 'Being' good on the other hand is being good even when there is no selfish benefit to you at all - and you know it. Being good is doing the right thing even when you know that gossip, tit for tat, getting something later, is not going to reward you for doing the right thing.
In evolutionary terms, creatures who are good and are not just 'acting' good are at a large disadvantage in passing on their genes. A person who donates money to a charity in front of everyone may get a selfish benefit from it because people will respect them. But a person who gives a lot of money to a charity without anyone knowing will probably hurt their selfish interests. Enough of those sorts of decisions will hurt your chances of reproduction. So evolution works against people 'being' good rather than 'acting' good.
This is why people are selfish in evolutionary psychology - evolution made us act good, but we are not actually good. In evolutionary terms, everything good we do is for the sake of appearances, higher social status, and later benefits (with the exception of kin selection).
Original sin: people have a 'good essence' in the image of God, but we are enslaved to 'selfish self-interest', which comes from being able to reason and think critically about how we could get ahead in life
In original sin, people have a basically good essence in that we are all made in God's image. Just like God is love, so humans were meant to care for each other and God in the way that God does. We have a moral sense that holds us accountable to the way the 'image of God' should act. But we are also pretty clever and we can tell that if we mess other people around then we can get ahead in life. This isn't the way we were meant to operate, but it's true that being selfish can get you ahead in life. So even though we have a good essence, with free will + being able to think of how we could pursue our interests without regard to others = you get original sin.
We have this devious ability to think because everyone has chosen to rely on themselves for knowledge of good and evil rather than God (although only Adam and Eve literally were in such a situation, we would have all made the same choice, so we are all 'in' Adam).
What God wants instead of this situation is for all our moral decisions to be made on the basis of 'faith in doing the right thing', which in practice, we cannot consistently do. In the Christian view, humanity has free will and thinks in terms of 'rational self-interest', which often leads to selfishness - humanity is not made selfish from evolution. Evolution is how humanity was created, but it never gives us a 'free pass' when we do the wrong thing, because it's not why we do the wrong thing. We do the wrong thing because, out of a devious understanding of self-interest, we ignore God's command to do unto others.