My primary avenue for art is in writing. I enjoy it quite a bit, and I like to think I can do it well.
I still feel as though I have not earned the right to call it art. I always refer to it as "my writing" or the like. It's not, and will never be to me, truly artistic.
But that's fine. I don't need to prove that my writing is art to enjoy doing it. I don't need to prove it to myself and others that it's worth doing. The fact that I do write, and that I enjoy it, is all that I need.
Arguably, it's the definition of genocide at play here.
To qualify what I am going to say, I have a minor in History, with a particular focus on the 20th century.
There are moments in both the Soviet Union' and China's histories that are genocidal, or aren't always considered genocide but probably should be considered it. Things such as the Holodomor, etc (I'm not going to argue if this is a genocide or not). As you mention, both nations likely killed more people than the Nazis did (although things such as the black book of communism should not be considered a credible source).
The difference is, neither the Soviet Union nor Communist China were founded on a platform of genocide. The Nazis were. The majority of people killed in the Soviet Union and China were not killed through genocide, they were mostly killed through political violence and state mishandling of resources. The intentions were just different; the Nazis cannot be understood without their desire for genocide, the Soviets can.