It was a car. It wasn't yours, yet you thought threatening violence was the appropriate answer. You were the agressor and bully in that situation. In that situation, if you had said that to me, my answer would have been "Feel free to try". I don't respond well to bullies or threats.
Yes, it was a car...a car that didn't belong to the jerk who was leaning on it despite the owner asking him not to do so. Chances are good that a threat of violence is the only thing that would get through to a punk like that. Unlike you, however, he realized that it wasn't worth getting his ass whipped.