The Western democracies were certainly right to fear what Stalin was doing in the years after 1945.
First, they were right to fear him because he was the leader of a country that was committed to an explicitly expansionist ideology. Communists believed that the whole world would become communist and the Russians felt that they should lead that world. It was therefore logical to worry that Stalin might try to expand by simply conquering other countries.
Second, there were a number of things that Stalin did that could have confirmed these fears. One major example was his push for the Soviets to develop nuclear weapons. A second example was the Berlin Blockade. A third was his support for the communists in the Korean War. All of these things seemed to indicate that Stalin and communism were acting in very aggressive ways. It was, therefore, logical to worry that they might truly expand if they were allowed to do so.
For these reasons, it was logical to fear Stalin in the years between 1945 and his death in 1953.