*And then the lawyer in me feels the need to quickly disclaim that the above question is obviously facetious, intended to pique interest rather than to suggest that genocide is anything other than heinous. That said...
Genocide is really the prototypical evil thing, both in terms of convenient rhetoric for a lot of debates and for international law purposes. And I agree that it's pretty darn evil. But I'm not entirely sure what about it is more evil than the murder of an equivalent number of people based on some other grounds. I should note that I also have some instinct that this is the case, although perhaps not so strong an instinct as is common.
Okay, so is it a question of mens rea- the mental state of the genocider? It's less moral to kill somebody based on ethnicity because it compounds the offenses of murder and of racism, so assuming that racism has a non-zero evil quotient, the same murder will be x + y evil instead of just x evil (presumably the justification behind hate crime legislation as well). Although if we are accepting this as mathematical in any sense, at some point the genocide of a relatively small nation would be less evil than the non-racially-motivated murder of a whole lot of people. Which I'm not sure is the case, but it would be interesting to run experiments to see how people feel about that. However, I don't think it's just the compounding effect of racism. I don't really know the full legal definition of genocide (and I'm pretty sure that there's not a simple or uncontroversial one) but racially-motivated mass murder may not qualify unless there is some real goal of annihilation. (Interestingly, I'm not sure mass murder based on other discriminatory patterns- murder of gays or the handicapped or lefties- would count as genocide, even if the goal was annihilation).
This implies that the real concern is something closer to the value that we place in diversity. Perhaps a better analogy than hate crimes would be endangered species. We think that there is value to the continued existence of variety and diminishment of said variety is a real harm. This is all very well and good, but as Rachel (my older sister, for the, like, two readers who are not family members) has pointed out, the value is never to maximize the number of animal species. In fact, those who are most concerned with conservation of endangered species also tend to be those who are really offended by the idea of frankenfruits and so forth or any sort of human-created diversity. And back to the human example, I don't know of that many people who encourage more people to go split off and create new ethnicities or cultures or what have you. Although the general lamenting of globalization and so forth is prevalent, I'm not sure anybody thinks it's as evil as genocide. Is the difference intent? Or is genocide precisely as evil as mass murder + globalization?