What would motivate someone to run over a dead cat?

Details: Line of cars traveling down an access road, one of the cars swerves out of the main stream of traffic in order to run over an obviously dead cat that is on the shoulder then swerves back into line after accomplishing said maneuver.
Why?
What benefit either real or imagined did this person get by flattening an already dead animal? Is this an indication of deeper psychological problems? Is there a difference between someone who would swerve to kill an animal and one who “re-kills” one? What is the internal motivation behind such an act or is it a lack of something in the psyche, like compassion maybe?