Little Bill has just discovered that his wife is, yet again, getting fucked by another man. Instead of angrily telling his wife to stop (and, subsequently, getting told to fuck off) as he did in previous scenes, he instead calmly leaves the house, walks to his car and and pulls a gun out of a compartment. He casually loads the gun, walks back into the house, wading through the crowds of partying people, opens the door to the room and shoots his wife and her lover dead before calmly killing himself.
The tracking shot isn’t “fancy” or “flashy” in a traditional sense. The camera simply follows every step Little Bill takes, every action he makes. In previous scenes, Little Bill always makes his anger and discomfort clear, but it is always treated very humorously. In this scene, the calmness of his actions gives the entire sequence a sense of extreme discomfort and tension, and just what is happening isn’t clear until he actually loads the gun.
It is simple and to the point, and its simplicity highlights the utter coldness and creepiness of Little Bill and his revenge. The fact that Little Bill doesn’t speak just seems to make the whole thing more unsettling, and it properly sets the tone for the new decade ahead of the characters.