Marquis of Roland, unlike others I`m not going to belittle your opinion or get personal. I think it is an interesting comment that deserves discussion. Saying that an opinion is preposterious is easy and it never serves the discours.

I am maybe the biggest advocate of ethical relativism on this forum and I made a challenge out of it to convince people of it. That`s why I take objections at your post.
You have correctly asked to define good and evil. The question whether the Soviet Union was evil is of course dependent on the definition of the term 'evil'. So much should be obvious. But I note that you do not provide a clear definition of evil as well. Yet, you make evaluative claims. That seems to be an inconsistence in your position.
Ethical relativism allows evaluative statements if the moral assumptions of the discours are explicited or equivocale. Neither is the case for your position.
Your post seems to imply that you handle an implicit definition of evil, since you refer to actions that would be irrelvant unless taken as examples of evil. It is rather weak to criticize others for not explicitating their definitions, when you don`t do it yourself.
However, the bigger problem with your implicit definitions is that they don`t seem to be common. Since your arguments are enthymemes, others have a hard time to follow your reasoning. For example you say:
"Really, the Nazis were no more evil than the United States. United States was no more evil than Great Britain. Great Britain was no more evil than the Soviet Union. "
It`s not clear how you come to this conclusion. You correctly name acts done by these entities that can be seen as evil under a common implicit definition, but the common definition would also entail that there are degrees of evil. Thus, the fact that GB and the US have done evil does not imply that they are just as evil as the Soviet Union or the nazis. Rather under the implicit understanding of evil I would use, these examples provide room for the Soviet Union and the nazis to be much more evil. Influencing other people`s culture seems to be a lesser evil to the systematical marginalization and extermination of millions of people. If your reasoning is correct under the premise of your implicit definition of evil, it is still unclear how that is possible. Ethical systems under which MacDonalds is as evil as concentration camps may be conceivable, but they are hardly widely used.
So in general, your position is either wrong or it depends heavily on premises you do not explicitate.

I also have to note that you make not a single point that would indicate that the Soviet Union was NOT evil. All your points about the US, GB or Christianity are ignoratio elenchi, as their evilness and that of the Soviet Union can be seen as independent. So I would like to restate the question: was the Soviet Union in your opinion evil and if not, why not?

The way you phrase your argument makes it seem as if what you say follows from ethical relativism or at least from the openness of the question how evil is to be defined. I would like to remind everyone here to note that this is certainly not the case.