I think that stating that humans are inherently evil is an inappropriately strong statement. As a lot of people have mentioned, inherently self-serving would be a better phrasing.
Evil to me indicates that you are performing an act solely for the destruction/carnage/hurt feelings it will cause. Self-serving merely means that you would perform an act that caused destruction/carnage/hurt feelings IF you believed it would benefit you in some way.
Characterizing humans as self-serving is a somewhat primitive characterization of us as a species, but as has also been stated, there is some evidence to support this view.
I tend to view the whole thing as a continuum between pure altruism and pure self-interest (not pure good and pure evil), where being too far along in either direction is a bad thing. (Disagree? Think of someone who is so altruistic that they give away everything to others - including the food and shelter they need to survive. Pure altruism is the recipe for a very short life.)
I think, as a species, our "centre of mass", if you will, is a bit more towards the self-interest end of things. However, as in any population, there will always be variation from the average.