At different points in the story, it's revealed that John is an alcoholic and that the fur gun's effects are based on the wielder's thoughts. When John first fires the gun, it makes a beer larger, revealing that John really wanted a drink at that moment.
A recurring theme of the book is whether or not the infected will inevitably "monster out." Although they never explicitly say it, many of the major characters know that at the end of John Dies at the End, David was been replaced by a different kind of monster who hasn't "monstered out" yet.
