First of all, they did not (facts), they signed a non-aggression pact to bide their time while the West refused to ally against the Nazis, hoping that the Nazis and USSR destroyed each other.
Second of all, the Soviets were true Communists and established a Dictatorship of the Proletariat as is in line with Marxism.
Please read theory before you cosplay as an expert in what is and isn’t “true” Communism, it’s painfully clear that the furthest you’ve gotten is Wikipedia.