Soviets exterminated scientist, priests, doctors and officers - the very people who could fight back. They integrated poor and weak. Their idea was to melt the remaining people into one nation. This is a form of ethnic cleansing.
Both Nazi and Soviets had the same goal - to take over the world by killing the people they didn't like and integrating people they needed. They just had difference of opinion in show to achieve their goals. Fortunately they didn't succeed, unfortunately it still resulted in millions killed by both sides.