It is a case of hyperbole, as a result of the extra scrutiny he faces as a result of being black and democrat.
Sorry, there is no non-political answer to this question.
The reason I consider him a tyrant is because of so many innocents being killed in Pakistan everyday in drone strikes and he is doing it much more than Bush ever did. But, I don't think many Americans think of this when they call him a tyrant.
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