In games like Sins of a Solar Empire one billion is small time. Even in less massive games like Shogun 2 Total War I put over 200 hours in which has got to mean I've killed the entire population of Japan at least once. Let's add in Warhammer 40K Space Marine which leads to the deaths of thousands of orks by my own hands, the entire total war series, Sword of the Stars, Prototype, the Saints Row and GTA series, modified Dawn of War Dark Crusade (sweet mother that was a lot of violence), Master of Orion 2 (you wipe out one of my planets, I wipe out all of yours), Civil War Generals 2, Defcon (a great game for boosting your kill count), the civilization series, etc etc
TL;DR Add a "trillions" option