I read an essay by Stephen King where he claimed that his writing was basically socially conservative and morally simplistic - there's always evil in his worlds, but it's always an invader from the outside that must be repelled.

That seems like a major oversimplification. A whole bunch of exceptions spring immediately to mind, such as pretty much all the Bachman Books (where the villain is often society itself or the masses thereof), and short stories like Dolan's Cadillac (where it's not really clear who's the bigger villain). And what about Firestarter?

Even in books like The Stand or Needful Things, where the evil really is a non-human invader from the outside, it gets big chunks of its power from individuals' failings of character.