Somewhere in the billions, I think.
I haven't blown up that many planets that I can remember, to be honest. The only time I can recall doing it was in Colonial Conquest II on the Amiga, but planetary populations rarely exceeded 35,000 in that; I've clocked 455,410 lifetime kills in Diablo III, so that dwarfs that one.
However, I've played a little DEFCON, which comfortably puts me up towards the billion death marker. Not a lot, so probably not much more than that in terms of orders of magnitude.
Wolfenstein 3D has some 4,000 Nazis. I've killed all bar three (the Hans Grosses in the Aardwolf maze in E2M8, because fuck that shit).