I've been reading a lot of H.P. Lovecraft and related mythos tales lately. It's fun to sit down and begin a story with a thought like, "Such quaint prose!" and end it with the heebie-jeebies. I've realized, however, that Lovecraft's stories aren't as scary if I try to adopt his own philosophical presuppositions when reading them.
Lovecraft was, as far as I can tell through my limited research into the matter, a materialist. Well fine. But if this is true, then Cthulhu is nothing more than a squid-headed dragon-man from another planet. He may have an unknown physiology. He may even be composed of unknown elements. But he is still simply a material being, nothing more, nothing less. Thus killing him involves nothing more than applying the proper amount of force to the proper place in the proper way.
Now, weapons technology has, for better or for worse, increased in leaps and bound throughout the twentieth century. How long until we can turn great Cthulhu into little more than cosmic dust and sink R'lyeh forever? How many nuclear bombs does one think it would take to reduce the mountains of madness to the plane of glass? Or to seal the deep ones forever in their undersea trenches? Demons are scary because they cannot be defeated by force of arms. But aliens can be. It's just a matter of discovering, stealing, building and/or modifying the right technology. Heck, large dogs could wound and kill the fungi from Yuggoth, how scary can they really be once one gets over their unfamiliar appearance? Pluto's not even a planet anymore, why should Terra fear its inhabitants?