Obviously most people's first instinct would be to say "Nazi Germany". But the more I think of it, the more accurate this list seems to be:
10. The Ottoman Empire
9. The Spanish Empire
8. The People's Republic of China
7. Confederate States of America
6. Belgium (under Leopold II)
5. Democratic Kampuchea
4. Soviet Union
3. Imperial Japan
2. Nazi Germany
1. The British Empire
The criteria were:
- Body counts, of course, but not just the numbers. The percentage of a population killed and the reasons for doing so also counted
- The reasons for founding the particular entity; this probably boosted the CSA, for instance.
- Wars of aggression, including their scale and motivation.