It’s not about whether a genocide is right or wrong. All genocides are wrong, and should be prevented when possible. The point is that if there are two sides to choose from, realistically speaking, the fact that both engage in genocide does not make them equal. There is generally still a lesser evil, and oftentimes by a large degree.
Again - if it comes down to the wire, between the Nazis and Soviets, is there a lesser evil there?