You're spot on. Most cold war propaganda about the Soviet Union is regurgitated directly from Nazi propaganda. It's quite simple really. Always suspect what biased people have to say about anything. Including Communists.

Stalin was undoubtedly a harsh man but consider if he was not harsh? What if Stalin had been laxadaisacal about the industrialisation of the Soviet Union? The truth is the Nazis would have butchered 100's of millions of people rather than than the 20 million they got away with before they were destroyed.

In regards to the purges I mostly put the crimes of that era down to internal sabotage and mass hysteria. Stalin definitely shares some of the responsiblity. But I would argue that some purge of the Communist party was necessary and inevitable.

Here's a decent video series in regard to the purges if you want more information.

Follow the sources referenced if you want more information or just research about it here.