Story, first published: Astounding Science Fiction, March 1943
In "Shadow of Life" ..., aliens who believe in the "rightness of evil" recruit Earthmen to preach their doctrine. Eons earlier, the aliens have already scared the old, arrogant Martians into hiding in a sub-atomic universe. The humans find a weapon to fight the aliens and want the Martians to cooperate, but the Martians refuse. Only humans, the younger race and more adaptable to change, remain to take on the Evil Beings. This is one of the few stories in which the aliens are "truly" evil, but even then, Simak justifies their actions because of their creed of "no room for benevolence in the Universe."
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.28