Factual error: During the first mall robbery, when Marcus is sliding down the middle of the escalator, both sides of it are running. They would have been turned off as the mall was closed.
Factual error: The last half of the movie is supposedly set in Phoenix, Arizona. Since I live there, it was obvious that none of the shopping centers, housing developments, street names, police car markings, or police uniforms were from Phoenix. They looked like they were possibly from Los Angeles.
Answer: My guess is that when he was a child, he was molested by a man dressed as Santa Claus, and ever since then he has been holding a grudge against people dressed as Santa Claus. You can see him staring angrily at Willie in the bar even before Willie notices him.