From another Reddit discussion:
“What would each leader have done, had they the unfettered means to do so? Stalin would have liked to simply rule unopposed. His actions were about the elimination of opposition to engender dominance and power over a homogeneous populus by any means necessary, and without targeting specific groups for persecution (arguably). Partly this was a deterrent; one the one hand, to prevent his people from opposing him, and on the other hand, to terrify other nations, by showing them how savagely he treated his own, and therefore inviting them to imagine how he’d treat his enemies. It was political, not racial.
“In the case of Hitler, it wasn’t a matter of opposition. While political dissidents didn’t have it easy, the major proportion of deaths were fuelled by the belief that Aryans were innately, inherently better than all the others, and that legitimised the systematic persecution, oppression torture and eventual destruction of others, be they handicapped, the gypsies, LGBT, or Jews. Therefore, had he had his druthers, it all would have ended with this world being utterly wiped clean of those people he regarded as “racial chaff”, irrespective of whether or not anyone opposed him. It was the calculated removal of an entire ethnic group or race, otherwise known as genocide. And while mass murder is obviously heinous and evil, there is something far more grievous and disgusting about attempting to obliterate an entire group of people purely because you think you’re better than all of them because blue eyes and blonde hair, and despite the fact that Jesse Owens ran faster than all your Aryan athletes at the 1936 Olympics in Berlin.
“Also, while Stalin killed lots of people, Hitler killed about 2/3 of the Jews living in Europe . And considering he never had a chance to obliterate the English Jewry, that’s an incredibly effective genocide of the Continental European Jewry.
“So, why do we react so badly to one and not the other? Because in order to decide to kill every Jew in Europe, or enslave every African in the US, you need to deny their humanity. You need to, somewhere in your mind, decide they are animals, vermin, insects, and that you have a right to treat them this way. Political murder is a means to an end, which is evil, but does not imply the same perversion of humanity. Whereas genocide is rooted in a fundamentally morally sick and evil belief that some imagined narrative actually gives you the right to obliterate others en masse, and is not the means, but the end.”