So is it harmful to focus on our alleged selfishness?
Yes. It is extremely dangerous. Many economists are great believers in the idea that everything in nature is competitive and that we should set up a society which is competitive to reflect that. Anyone who cannot keep up, well too bad.
I believe that is a total misinterpretation of the facts. The individual is not all-important. Yes, we can be selfish but we are also highly empathetic and supportive. These features define us and should be built into society.