It's well-accepted that Stalin's various purges and famines killed several times more civilians than Hitler managed to, for instance.

Here in the US, I'm personally more concerned about a replay of Pol Pot's regime. He took Hitler's warped notions of anti-intellectualism ("Jewish physics") to a whole new level ("Kill anybody wearing glasses.")

The fact that he did so under the standard of leftism doesn't really seem that meaningful or relevant, because if/when it happens here, it will evidently be perpetrated by the extreme right. A pox on both their houses.