I've had an ongoing idea for many years now that is more or less this idea but done through viruses. My idea would either sterilize or kill (mostly this one, since sterilization through disease/natural paths is very difficult).
Creation of viruses that focus on specific racial/ethnic groups, such as Ethnic Han Chinese, Jews, Hispanic origin and Sub-Saharan Africa, along with Arabian. It would be released through the water supply over a period of weeks, nationally and only in Europe, USA, Australia, Canada, and New Zealand, along with South Africa and Rhodesia if possible.
While some of the above countries have water systems that can be easily infiltrated to dump a few 55 gallon drums at a time, other do not, and would need to be spread through the air, similar to the way that they would crop dust for farms.
While not a virologist or heavy background in science, I would imagine that the flu, the common cold (one of the strains at least) and chicken pox, could all be used as means of infection, especially since these viruses spread quickly and their onset may be delayed.
But the best way would be to do all of this under the radar, one way would be to start with clubs, churches, gas stations, rec centers, especially parks and basketball courts, for blacks. These areas are highly concentrated with them if you introduce just one or two people in these places, then you can spread the viruses nationwide or at least throughout a city or state.
Just some ideas, I won't expand into detail for all of the races but you get the idea.