How is it terrible compared to other similar in scale countries?

The Soviet regime was so terrible that they had to build a wall to keep people in.

https://en.wikipedia.org/wiki/Berlin_Wall

The other similar in scale countries have performed much less atrocities against their own population.

China has a bit larger total numbers, but they're not similar in scale.

I don't get the logic: if you do it, you are a terrible person. If we do it, we are fighting for freedom or some such.

If you use the term "whataboutism", you are either ignorant or a shill.

If we disregard the geopolitics and foreign interventions, cold war UK, France or USA was a decent regime to its own people.

If we disregard the geopolitics and foreign interventions, cold war USSR was a horrible regime against its own people.

There are all kinds of horrible things done by those regimes to their own people as well, but they aren't comparable in scale.

It's terrible no matter who does it. You tried to claim that it was less terrible for the USSR to do it, because other countries did it too.

Worse than no justice is only selective justice: one set of rules for us vs. another set of rules for them

True, but no justice is still pretty bad, and you're advocating for it vigorously.

Maybe the fact that it murdered more people than Hitler's Reich?

In Bulgaria out of 8 million peple between 100000 and 300000 people were killed during the communist regime.