I agree with your premise for the most part as that is how I see good and evil as well. Pure good and pure evil mostly exist in theory as concepts. So they do exist, but only in a theoretical form. In practice, very few if any people can define themselves as pure good or pure evil. Almost everyone has done or continued to do bad and good things. However, that does not mean all people are equal or even close. I prefer to think of it as a spectrum.

Think of a stereotypical good person as being light grey where they are mostly good with some evil, a stereotypical evil person as being dark grey where they are mostly evil with some good and your average joe as being merely grey where they are roughly an even mixture of good and evil with shades in between all of them. A pure good (white) or pure evil (black) person is incredibly rare and probably do exist, but they are likely an extremely tiny minority if they do exist anyways.