YOu need to follow the laws of each state, ethics dont play into it....Here in Texas it is against the law to touch a game animal that has been hit by a car, there is part of the law that allows you do dispatch a animal that has been hit on the road, BUT the law enforcement officers can still write you a tickit, and the law was designed for a defence in court...

And yes I too have seen people sawing horns off of road kill buck, even if it is against the law in Texas..