Smart people don't think others are stupid
merely because of the beliefs they hold. That is, being a [Southerner, Northerner, Republican, Democrat, Indian, American, Christian, Muslim, Atheist, Tauist, Piist, vi user, emacs user, ...] does not mean a person is stupid. There are smart people who really have thought things through and ended up at each of those positions for good reasons.
But smart people might conclude some specific individual is stupid because of consistently erroneous patterns of thought and analysis which do not improve with experience. There are a few individuals who I've spoken to on a variety of topics over the course of years who, as a rule, develop beliefs based on sketchy evidence and then retain those beliefs even in the face of mountains of contrary evidence. Even when they hold essentially the same views as I do, it's often for bad reasons; there isn't really a coherent thought process or a body of evidence that got them to that point. Such people really are stupid.