Sometimes, employees of certain (private) companies tend to go too far while dealing with a client that it gives a bad name to the company. However, in many cases the company management are not aware of such situations, and I am sure they dont agree with their employees as well.

Approach a responsible person in the car rental company and tell him your position. You need to clearly communicate to him that you do not intend to pay anything to repair the damage.

That said, I am sure he will figure out a way to take it from there!