Is it legal to kill your roommate's cat???
There are multiple ways to deal with the cats. I've got 4 cats and a dog. The cats are awesome at keeping mice and rats out of the property, and the dog keeps everything else out.
Ask your room mate to help with the cost of materials for you re-painting.
I did see one of my cats come sliding down my 05 neon leaving a slight trail of claw marks. A good spray with the hose, and he never got on the car again. A little wet-sanding and some buffing cleaned up the paint.