This is not a terribly intelligent argument. Nobody claimed USSR was some perfect society, and I'm frankly not aware of any human societies where atrocities don't happen. Humans do bad things regardless of what system they create. Thinking that anarchist society would be all ponies and rainbows if the height of idiocy.

What USSR should actually be judged on is whether the system overall was an improvement on how people lived previously. This is a fact that only a person who lacks any intellectual integrity would deny.