The reason why the Muslim extremists are called terrorist and this other guy isn't is obvious: The purpose of these Muslim fundamentalists is to instill fear of within the western world. They hope that because of this fear the Western nations will do what they demand. For every act of terrorism there have been mulitiply threats of violence.
This other fellow so far appears to have worked alone. The reason why he did what he did is still a matter of speculation, as is his mental state. But unless we find an organization behind him who supported his crime we have no reason to believe it will likely happen again.
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