Genghis Khan is up there. His conquests killed millions, I see estimates of up to 40 million, a significant percentage of the world's population (possibly double-digits). It's even theorized that so many people died that global temperature dropped as a result. You could go and argue that a large unified empire would prevent many future wars and thus could be a net positive even if establishing it is very bloody (see Pax Romana), and Genghis' rule was reportedly relatively progressive compared to his contemporaries. But then, you need to make it so that the emprie doesn't immediately break apart after its ruler dies, which he failed at.

Though you can always argue that he wasn't really more evil than other rulers, just more successful. Which still makes him a "great villain", but there are more directly evil deeds than conquest, such as genocide.