"We formed an alliance with Stalin right at the end of the most murderous years of Stalinism, and then allied with a West German state a few years after the Holocaust. It was perhaps not surprising that in this intellectual environment a certain compromise position about the evils of Hitler and Stalin—that both, in effect, were worse—emerged and became the conventional wisdom." — Thimothy Snyder, a Professor of History at Yale, US.

An interesting perspective.

Stalin vs. Hitler: who was worse?

Who killed more: Hitler, Stalin, or Mao?