Answer:
The possession of nuclear weapons by Russia, and the perceived infiltration of Communists in government -are two reasons why Communism was seen as such a threat to America in the 1950s.
Explanation:
Short answer: violent revolutions that typically ended with the murder of most of the existing elites and confiscation of their properties, and the establishment of repressive police states that would either execute anyone or banish to gulags or work camps anyone who disagreed with the Communist Party.
Long answer: when communism first came up as a theory, it was a seen as the natural evolution of society by its fans - feudalism to capitalism to socialism to communism. With the advent of mass democracy, it was seen as only a matter of time that the workers would organize enough to use their greater numbers to force change in the system and move capitalism to socialism (and eventually communism).
In 1871, after France lost the Franco-Prussian war, the people of Paris weren't too happy with how it went and established a new government (the Paris Commune) with a fair amount of support. It was a much more communist government. The "national government" fled Paris and set up base outside the city. The two argued. Then the national government vowed to crush the commune, and issued orders to execute anyone holding a weapon or with gunpowder on their hands. Over the next two months, the revolution was bloodily put down and there were mass executions of almost all the major figures of the revolution.
The communists realized that the middle class would ally with the elite over the workers when push came to shove. The bolsheviks (Russian communists) decided that if they were able to have a revolution, they needed to crush any potential opposition or they'd get crushed themselves. So in 1917, that's just what happened, leading to the millions dead in the Russian Civil War.
Of course, once you start killing anyone who presents a threat to you, it is hard to stop. Leading to Stalin's great purgers and the Holmodor, and the various campaigns of Mao in China when they took power. Also, refusing to hear dissent allowed mass famines like the Holmodor and the Great Leap Forward.
The successful Communist revolutionaries of the 20th Century generally subscribed to "those capitalist swine will be the first ones against the wall when the revolution comes" - which certainly put a scare into the elites in the West.
