Impossible to say. The Mass Effect franchise easily nets about 605,000 kills (almost entirely because of Arrival) and in the various Total War games I've racked up several million kills largely due to sacking cities in late game rather than simply trying to integrate a population into my ranks. Various FPS multiplayer games tend to net 10,000 kills each so easily 300,000 kills there. Some 4x games likely would have staggering kill counts but, as they tend to abstract population, it is impossible to say. I will say that in terms of explicit kills, the only game truly worth counting is DEFCON considering single games can easily result in hundreds of millions dead and across all my games I've easily gotten in excess of 10,000,000,000 kills.