In light of the recent events of Charleston, I just wanted to make a quick point that's often lost in the analysis of racism more generally - take a quick glance at Roof's "manifesto" (you'll have to bear the broken grammar and recurrent spelling errors), and notice how central that concept of racial superiority is to the justification of violence.
I'm not sure racial superiority is the absolute beating heart of racism as a worldview (since you could argue that racism is just a variant of a more fundamental in-group/out-group distinction), but it's definitely one of the central cores. What's interesting is how easily the imperative to violence seems to follow once an individual has accepted the belief in White Supremacy as true. I'm not sure I have a good explanation for this, but several come to mind.
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Bret Easton Ellis