Over 50 Million people have been murdered by government decree in this past century alone, so it is difficult to break down who has killed the most. My best guess would be the Catholic Church; technically the Vatican is its own country. The Crusades and Inquisition obviously claimed millions of lives. What's more, the Church effectively halted the scientific advancement of the Western world for centuries because it was "heresy" to deny Holy Scripture. This is why scientists lost much valuable information over the years and ultimately ended up "rediscovering" things that were known to the Greeks thousands of years before.