The reason I am vegetarian is that I do not want to contribute to animals’ death or suffering. Therefore, I have no objection to eating an animal that was accidentally killed by a car, provided that doing so would not encourage anyone to kill other animals in the future.
Still, although I have no ethical objection, I choose not to eat roadkill for a variety of other reasons. It is like having no objection to eating rat droppings, as long as doing so does not harm them, while still preferring not to for many other reasons.
More answers: Vegan Ethics FAQ.