The president can do jack shit to stop a state prosecution, one of the blessed advantages of federalism.
I just don't hold much faith in the separation of power between state and federal government, it sounds like there has been a massive erosion of this barrier since the civil war and the abuse of the commerce clause and all that.
They will certainly attempt to dismiss any charges, but they are far from guaranteed that they will be successful.
So is it part of their official duties to walk in front of a car of someone who is trying to leave the scene, alter their path when the car turns out of the way to ensure they remain in their way, and then shoot the driver? Or is that merely the kind of excessive force that's in contravention of their training and not part of their job role?