If you were a Russian, Stalin was very bad, but Hitler was worse. Yet, it took Hitler proving it, to make the typical Russian a true believer.
Katin (and other Soviet atrocities) occurred before Hitler invaded those areas, thus it set a tone going forward in regard to the Soviets. That said, Hitler immediately went to work enslaving the entire populations of those lands, forcing the women into brothels, and exterminating certain elements altogether.
A very, very bad time to be in Eastern Europe.
Stalin knew many Russians preferred Hitler to him, so he went about a campaign of terror to force their loyalty. Katyn occurred when Hitler took western Poland and Stalin took eastern Poland.
It was a miserable time for eastern Europe; the fortunate ones in 1945 ended up of west of the line Stalin and Roosevelt drew between communism and relative freedom.