Personally, I don't think that ANY ethnic group will ever replace the Nazis in terms of sheer "kill as many times as you like without even a blip on your karma meter", as no other ethnic group has ever had the right mixture of a completely nutso leadership and cultural identity with a military powerful enough to spark a global conflict. The really nutty countries (Iran, North Korea, etc.) are mostly isolated nations who are more content to make empty threats to the world than actually carry them out, and even if they DID go to war, they'd only get as far as a few neighboring nations before being carpet-bombed to the stone-age by the US (i.e., Saddam's Iraq trying to invade Kuwait).
Historical empires are far trickier, as they actually balanced out the same genocidal practices of Germany with, y'know, actual empire building. The Roman Empire is probably just as bad as the Nazis (hell, fascists got most of their doctrines from the Romans, especially Mussolini), but the fact they conquered their known world AND built modern European civilization as we know it means we'll still remember them for their badass praetorians. Hell, even the enslaving of races for manual labor was a step up from massacring them for a scientifically implausible definition of being "genetically superior", and even the gladiatorial death-matches weren't always death-matches in reality (and even if they were, it was still more civilized than straight-up execution).
The Mongols are another example of being like the Nazis, except they succeeded where the Nazis failed. This is especially since they actually have an underdog, "Noble Barbarian" air about their history: going from a bunch of nomads roaming the northern border of China, to taking OVER China and Asia, and large parts of Europe and the Middle East. The sacking and razing of their enemies was brutal, but they were actually pretty decent to live under if one accepted their rule.
And aside from being more wildly successful than the Nazis in the "take over the world" business, the atrocities of the ancient empires are more tolerated today since ALL ancient empires did the atrocities - it was just what was commonly accepted then. By the time the Nazis went about their genocidal rampage, the civilized world had largely gotten over the barbaric practices of ancient times (and I mean ALL the civilized world, not just the west), which made the Nazis devolution to those levels genuinely horrifying in the modern, industrialized era.
The closest equivalent to the Nazis in World War Two I can think of is Serbia in the Yugoslav wars, and it's still a mostly regional conflict without world-threatening proportions (although it's still sorely disappointing that there aren't as many games out there about hunting down Serbian war criminals as there are about shooting up Nazis, Russians, and nondescript "organizations" that give plausible deniability, what with being the era of political correctness and all).