I guess it is really not until we get to the prequels that he really becomes complex. Every villain has a back story (though I suppose that isn't always true), the fact that he sired the hero and was a one point good and becomes good again before allowing his son to pull his life support (which is literally all we really know about his complexities from the OT) doesn't make him all that complex. I guess it is when we get to the PT and we actually meet him that his complexity really starts to show, as now we have three movies worth of backstory for him.