Its gotta be Asia, though I'd guess Europe is a close second. Who gets Stalin? There's an argument for attributing him to both Asia and Europe.
I wouldn't say Europe is a close second, especially because the Black Plague was attributable to the Mongol Empire who intentionally spread it in numerous cases for warfare reasons, or accidentally though trade routes. Stalin can be shared 50/50. Asia has Mao who was worse, as well as some pretty freaky dictators in history.