Why?? Because she could.
My lovely (socially acceptable, may I add) cousins used to make a game of scoring "points" by running over various animals with cars, lawn mowers, et cetera.
IRRC, domestic animals had a higher "point" value than wild ones, with cats and small dogs being particularly desirable as they were unlikely to damage the vehicle.
They once thought I was completely insane for turning around to put a groundhog I had hit out of its misery. It was writhing spasmodically in the middle of the road. I whacked it in the head with a tire iron until it stopped moving, then sat down, punched myself in the face, and smoked a bunch of cigarettes until I stopped shaking.
I'd have liked to hit them too, but that's not OK.
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"