I've actually been thinking about this every now and then the last few years. That in a not-too-distant future someone will be able to whip up a virus that can kill only those its creators want to kill.
Like you what I thought wasn't targeting individuals so much as ethnic groups, some white supremacist in the US targeting genetic traits mostly found in black Africans (and their recent descendants), European right-wing extremists going after Arabs, certain groups in the Arab world going after ethnic groups which are predominately non-Muslim and so on.
If I was one of these crazies I'd also take other factors into account, for example, certain cultures are predominately patriarchal so you could perhaps limit your weapon to men, thus not completely depopulating a region. Or how about some crazy cultists targeting women in general?
It's really quite scary to think of.