communism was deemed dangerous to the USA becasue it threatened her commercial interests as well as internal stability.
This is becasue communism promised such things that could only be achieved by a revolution and eventual installation of a proletariat dictatorship. This would bring to an end several years of struggle in making a democratic nation. In addition, communism threatened the trade interests of the USA in Europe and Asia. the loss of china to communism was a major turning point which led to a serious purge of perceived communists.