The accident itself may have been manslaughter (you never know with politicians - if she was pregnant he could have staged it deliberately as an excuse to kill her).
But once it happened and he got out, not trying to save her AND not reporting it for 9 hours raises it to second degree murder. Because there is no way he could not know that she WAS dying in there, and not providing or getting help IS deliberately aiding that death.
And of course if he did those things - in order - to deliberately kill her after the fact of the accident, then it's first degree.
After reading about Kennedy’s actions in the Damore book, I think justice would have been tying him to pole and having a firing squad execute him.
He just could not have been any more sleazy than what he was.