Our country chose an evil ally against an evil foe. The relative virulence of their respective evils was self-serving propaganda written into the history subsequently. Full advantage was taken, as is always done, of the victor's prerogative to write the history.
It would very likely have been possible to have manipulated the Nazis and the Soviets into fighting one another to their mutual destruction. But that would not have served the purposes of various politicians, Roosevelt in particular.