I think it has more to do with laziness than malevolence. You're damn right about King David. How about the Apostle Paul? Torturing and murdering Christians until God spun him around.
We're ALL a combination of good and evil. From my experiences, those who think they're "good people" are capable of the most heinous, depraved acts. The Nazi and Communist leaderships provide good examples of folks who considered themselves good people doing good things for mankind, and we know how that turned out. . .
Those of us who acknowledge the fact that we are sinners containing a vicious Beast within us and learn to keep that brutal f*cker on a short chain have a better shot at not doing such things, because we know about the horrid things we most certainly could do if we unleashed it.