I think we all agree that it was an accident and not her fault that she hit the guy.
If not, it was vehicular manslaughter. I think she should have been tried for that, and then tried for the acts that followed, separately.
What purpose would that have served? Nobody would gain from that except the lawyers and the court stenographer. The great state of Texas would have been spending a lot of extra dollars for a second trial to proove what? That she did infact let the man die.