The expected impact of an evil genius is often smaller than the expected impact of a wonderful genius. There are lots of would-be mass murderers and the bigger the population, the bigger the absolute number. But their ability to murder lots of people is limited by the fact that most people try to stop them. Yes, in some systems (Hitler’s Germany or Stalin’s Soviet Union), a mass murderer is able to enlist lots of people to help him. But that is rare. Most of the time, people try to stop them, and in some systems it is especially difficult to kill lots of people over any long period of time.I think Roberts is correct, though he is downplaying the fundamental impact of ideas in this struggle. The march toward statism in America has been slowed by the (however implicit) American political philosophy originally identified by Jefferson, Madison, and, ultimately, Locke. Conversely, it was the ideas of Kant, Hegel, and Marx that allowed Wiemar Germany and Czarist Russia to largely embrace the rise of Nazism and Communism.
Geniuses on both sides were responsible for these antipodal philosophies. It follows from your value judgments how you assign the designations of "good" and "evil" to them. You can't have it both ways (and I'm not arguing that Roberts tries to): the same person cannot logically label Locke and Hegel as "good" geniuses, or Jefferson and Marx as "evil." They're opposites. Given the full-scale bloodbaths that were unleashed as a result of the ideas of Kant and his ilk, I think "evil" is the only way to describe their genius.
What makes their evil so genius? It's that their philosophies are taken seriously and downplayed. Insomuch as they are studied in academia, the philosophies of Kant and the rest are seen as important, worthy of study. But their impact on the cultures of Wiemar Germany and Czarist Russia are largely skirted (at least in the popular descriptions of the rise of totalitarianism, and with some notable exceptions). It's this disconnect, that their explicit philosophies are studied and their impacts ignored, that makes them genius, however evil. It's why their legacies are still felt to this day.
This is why I think an evil genius has a greater effect ideologically, than he does in particular instances, like Hitler and Stalin. Hitler and Stalin, however evil and opportunistic, were not geniuses. As Hannah Arendt noted, they were banal. It's the ideas that spawned them, that bolstered their rise and political legitimacy, that must be combated, even more so than these despots who where their logical conclusion.
This is the evil that must be destroyed at its root, so its particular brand of genius does not endure.