Short Question

[Infernaw]

Shooting someone for bumping into your car is imo RDM.
If anything you can arrest the person for damaging your vehicle.

That depends Imo. If he continously bumps into your car, for no reason. I would shoot him, no doubt. However, like you said. If you are a cop, you should chase the person down.

 (Isnt it Quuuiteee FailRP to bump into someone on purpose though? Its not like the cars you have are one of those bump cars you find at amusement parks.)

To answer your question though:
1) Maybe, read what I said above.

2) Why would you return to him with a gun, when you know he is armed? And obviously, as long as he isnt under FearRP, he may kill you.

(Note: I am un sure, and you should get staff members opinions.. What I jyst wrote are MY OPINIONS.)

If it fits your character roleplay, you can threaten the person. But shooting blatantly and on the other hand, bumping into someone, is FailRP/RDM.
I recommend that if there is staff online, contact them so that they can stop the person from bumping into you and perhaps blacklist them off vehicles.