As people's souls become increasingly depleted spiritually, their souls becomes unguarded and demon powers have access.
              Luke 11:24-26 When the unclean spirit is gone out of a man, he walketh through dry places, seeking rest; and finding none, he saith, I will return unto my house whence I came out.
And when he cometh, he findeth it swept and garnished.
Then goeth he, and taketh to him seven other spirits more wicked than himself; and they enter in, and dwell there: and the last state of that man is worse than the first.  

People whose souls are empty and hungry will feed on anything that will give them satisfaction, even if it's deadly.