Personally, I take your question as a legitimate one, and I will try to answer:
The fundamental difference between the atrocities of the Nazis and that of the others you mention was intent.
Japan did not invade China and slaughter huge numbers of Chinese because their goal was to slaughter Chinese. The purpose was to establish a Japanese empire.
Russia did not cause a famine that killed millions of its own citizens because their goal was to kill millions of its citizens. The purpose was to massively industrialize their nation.
These actions were evil, no doubt, but the intent wasn’t necessarily so. Britain established an empire, and few consider that evil per se. The Japanese used evil methods to achieve a somewhat neutral end.
America industrialized itself, and few consider that evil per se. The Russians used evil methods to achieve this somewhat neutral end.
But the expressed purpose of the Nazis, as demonstrated in Mein Kampf, was to establish the racial superiority of the Aryan people, and to eliminate from Aryan society “mongrel” peoples who threatened Aryan purity. This was evil in and of itself. The Holocaust was simply putting this evil goal into effect. It is therefore qualitatively different.
To my knowledge, only three events in modern times compare - Cambodia, Rwanda and Turkey.
With the possible exception of Cambodia, these events simply differ in degree.
It is possible to argue, although I don’t, that Rwanda was qualitatively different. That argument would go as follows:
The Hutus, although the majority in Rwanda, were second-class citizens, and their intent was to end their second-class status. This is a neutral (or even possible good) goal. The method chosen to accomplish this, genocide, was evil.
It is also arguable that a qualitative difference existed between Nazi Germany, and Cambodia/Rwanda/Turkey in the willingness of the ordinary citizenship to go along with genocide. I’ve only read reviews of Hitler’s Willing Executioners, so I defer to Tomndebb on this.