1. Communists murdered their own people; the Nazis murdered others.
Add to that the fact that, unfortunately, successor governments in nations like Russia, and governments in China and Cuba, acknowledge no particular abuses of human rights. Using the Russian example, if they don't acknowlege the gulags for what they were, thus don't prosecute the persecutors, there's not much than can be done.
The families of the 20,000 Polish officers murdered at Katyn would beg to differ with that statement.