It may be easy to define some "hate groups" depending on what they do and what they say.
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But what if someone like Hitler defines a "hate group" as being "Jews" and seeks to exterminate them? 
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And don't claim that "Oh, that is ridiculous. That would never happen in the United States." Well, it happened in Germany.....and for many years the Germans, most of Europe, and the US turned a blind eye.....because it wasn't happening to them.