"The devil searches to and fro as a hungry lion searching for whom he may devour".
I believe that Billy's spiritual vulnerability is the unexplored weakness that could best explain why the landlady's plan is working so well. First I"m going to show where in the story these supernatural powers seem to be present. Then I"ll use a few inferences to show Billy's unique spiritual vulnerability and finish with a summary of some fairly obvious information about his personality. .
Billy's inability to walk away, after making a conscious decision to leave, demonstrates some sort of will-bending ability of the woman. (Pg 125 par 3) After Billy rings the door, the woman instantly answers; suggesting some sort of foresight. But despite having the seemingly innate ability to predict (pg 126 last par) and force Billy's arrival, the woman does not seem to be in full control. (Pg 127, par2) (She demonstrates anxiety regarding the delayed arrival of her next victim/pet.) Which is our first clue that the woman may not be the great evil of this story.
The landlady appears very nice and thoughtful. She is very loving to the point that I feel she is not entirely aware of the evil she commits. The story would suggest otherwise, but I think there might be some sort of trickery working against her. Perhaps she feels that her loving environment is the best place for her company. Whatever the situation, the point here is that the landlady is in fact, not the greater evil. Rather, I believe, some agent of Satan is at work in her mind and home. .
Satan has no power over any believer of Christ, for Billy to be compelled to enter, and be only dimly aware of the evil around him- he must not be a Christian. From what we can see of Billy's lifestyle his morals support such things as drinking (pg 125, par 2), swearing (pg126 par9), praise luck for good fortune (pg127 par7) and taking advantage of old ladies that lose their sons to war.