I don't think ClarkeIf Arthur C. Clarke is to be believed, (spoilers for Childhood's End)it's because devil-like aliens help guide humanity to their next level of evolution, but this leaves the last generation of humans in despair as all of their children are so far beyond them as to be incomprehensible, and the children are beyond caring about them. Even knowing that the aliens were only there to help, this despair is sent back in time as psychic imagery, resulting in the current demonic imagery.
Or, you know, putting down non-Christian religions.
described them as goat-like, did he? Weren't they more or less stereotypical Balrog-style demons?