I think he is the one who shot her!
Females have shorter arms than males, she was shot in the back of the head.... it would of been easy for him, to have his arm on the top of the seat and shoot her, from behind.
Rather then shooting herself behind the head... She would of gone for her temple, mouth or forehead...not behind the head, where was the hardest place to reach...
After all she was yelling at him and he probably got upset...
And he couldn't leave a eye witness after all...