I believe in Twisted Metal 2, the main character at the end wishes to kill every sentient being in existence, so that's going to be pretty high. As far as 4X games go (which have a natural inclination to high death counts due to dealing on the scale of planets and interstellar warefare) my first guess would have been GalCiv 2.
However, I believe we should make a distinction between death counts and body counts... body count suggests bodies on screen, which is quite different from a pretty picture of entire galaxies exploding. My first instinct on that would be the total war series, with extra large units; the restrictions and unit sizes for the last few games have all been rather similar, so I'm not sure which has the theoretical maximum, but with the reinforcement system you can get a lot of bodies on the ground.
However, I believe we should make a distinction between death counts and body counts... body count suggests bodies on screen, which is quite different from a pretty picture of entire galaxies exploding. My first instinct on that would be the total war series, with extra large units; the restrictions and unit sizes for the last few games have all been rather similar, so I'm not sure which has the theoretical maximum, but with the reinforcement system you can get a lot of bodies on the ground.