August Derleth created the "Mythos" of the Cthulhu Mythos, since he helped bring Lovecraft's fiction to world recognition by creating Arkham House publishing. Derleth, a Catholic, confused Lovecraft's intention by making the Cthulhu Mythos a battle between the forces of Good vs Evil, with the misunderstanding that the Elder Gods were the good ones and the Old Ones (Cthulhu, Hastur, Ithaqua, etc.) were the evil ones in a cosmic battle to rule the multidimensions.
Lovecraft, an atheist, considered mankind's role in the universe as insignificant in the vast reaches of space, and he didn't have the notion of Good vs Evil in his tales. The Old Ones were evil, from mankind's perspective, but he believed these entities really didn't much care what happened to humanity, since mankind was expendable. Perhaps it was this "uncaring" universe motif that made his tales frightening, because it removed any comfort zones humans liked to believe that the universe was entrusted into the hands of a benevolent, omniscient God or gods.
Cthulhu was merely one of these alien entities that Lovecraft created, sleeping in the sunken isle of R'lyeh, waiting for the stars to re-align themselves again in the heavens so that the gateways fly open again, releasing this creature and others to roam again among the stars. What is interesting is how many of the current writers of Mythos oriented tales tend to follow Derleth's Good vs Evil motif, watering down their tales with the hero or heroine going mad trying to save the world from these Bad gods. A few have managed to capture Lovecraft's effect...one such writer, from the past, was Clark Ashton Smith.
other links for more info:
http://www.necropress.com/catalog/
this site, if it comes back up again, has lists of tales by Clark Ashton Smith:
Clark Ashton Smith - The Eldritch Dark
Internet Resources this site had a listing of Lovecraft tales available for viewing, but many of the links are down. They may become available in the future...