Here is the thing you have to remember. Hitler and his Nazi goons actually thought they were doing the right thing. They thought they were doing good for Germany and Europe (this in no way justifies what they did). Stalin and the Commies on the other hand knew that what they were doing was evil and brutal. They did it because they did not care at all about any form of human life.
Like I said, not trying to justify what the Nazi's did, they were evil and brutal, but Stalin may have been quite a bit worse.
I think Hitler and Stalin were both brutal men, in some ways Hitler's killing was more illogical than Stalin, but ultimately it doesn't help his position much. Kill off all these Jews so Germany can be great again, or kill off all these anti-communists so Russia can be great agian? Both positions are ruthless. In some ways you can argue Stalin was more sensible than Hitler in not going to open war with the Allies like Hitler did, and that the Soviet Union lasted almost a 90 years, and the Third Reich only ten. Of course I wish the Soviet Union had not lasted so long, but it goes to show the craftiness of the Communists, something the Nazi's weren't the best at in their wreckless quest for power against impossible odds. The Nazi's couldn't wait, thus the terrible cruelities were quick and terrible, the Communist's were more patient, their cruelties were long and unending, but ultimately the Soviet Union sickened of something akin to a wasting disease. They were killed from within, no army marched in to conquer them.
It's pretty hard to put one method above the other, except I think the Germans were in some ways blessed to be rid of Hitler after a decade, while the Russians were cursed with Commie Dictators for almost a century.
The Russians have always liked a powerful leader, with absolute power, while in Germany, the Holy Roman Emperor was still called to listen to the Pope, and it was that way in Germany for hundreds of years.
I imagine both thought they were using the best methods for their country to achive power.
Oddly enough, Hitler is a far greater villian to modern culture than Stalin is. Left leaning commies in the classrooms perhaps?
At any rate, the Khemer Rouge sounds pretty bad, but I really can't give a vote in this case.