Story, first published: Marvel Tales, March-April 1935
In "The Creater" two scientist-heroes build a "timepower" machine that takes them to a super-universe where they meet a telepathic being who created Earth and the rest of the universe as a laboratory experiment. When the Creator attempts to destroy the primary universe (and hence all universes), the protagonist , with the aid of other aliens who stumbled into the Creator's lair, destroy the Creator and his laboratory. The hero-narrator is not rewarded for killing "God" and saving the Universe but returns to Earth millions of years later as the tribal chief and demigod of the last inhabitants of a dying planet.
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.24-25