Continuity mistake: The washcloth changes color and design when Willy is in the tub.
Continuity mistake: In the kitchen scene after Thurman cuts his hand, Willie turns off the faucet and then it's back on again.
Continuity mistake: The sidewalk to the front door of the kid's house has three (or more) tiers - one step up at a time followed by a landing of several feet, etc. After police shoot Willie in the back and approach him, the sidewalk to the steps before the front door is flat. (01:24:23 - 01:26:40)
Continuity mistake: When Willie is shot in the back by police, his feet land about midway between the two short brick pillars/columns (walkway to the kid's house). After neighbor kids are shown screaming and the camera returns to Willie, his feet are before the first pillar - several feet behind his landing spot. (01:23:45)
Continuity mistake: When the kid takes Willie the orange juice there is sun on Willie's face. Camera cuts and it's disappeared.
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.