That depends on how you define the population. Killing the worst 1 million people (people who have caused the most harm to other people, and would continue to cause significant harm) instead of 2 million random people would be a very large net benefit. There have probably been few or no traditional populations (nations, cultures, political movements, etc.) that would be worth completely eradicating, and probably never an entire 1 million people in such a population worth killing out of hand, but if I was forced to choose, I think I could find examples in the 20th century.