I have watched several videos and looked at Brian, his speech, mannerisms. I think I understand what type of a person he was. Just the personality type. I understand why parents found a lawyer - he is not a good speaker, either way, even when totally innocent.
I don’t think he intended to kill her at all, but I think he did. Unintentionally. The best witnesses of how the trip were was the policemen, and they registered that the couple was fighting. Sometimes during such fights, tragedies happen. This is why we have “involuntary manslaughter” charge.
Had anything else happened, he’d immediately called the police to help her.
Imagine this. A family member falls off the stairs. If it is a total accident, everyone calls 911, paramedics. If the family was fighting and the victim was pushed during the fight, even accidentally, the perpetrator thinks, “she has bruises. They won’t believe it was an accident. The police already saw us fighting. They won’t believe me”. And then his actions would not be unlike Brian’s ones - to provide an alibi (I was not there), to run away.
I think after whatever happened, he was hysterical and confused, and probably for a while, just ambling around, but finally, he did the only predicted thing, he ran (drove) to parents.
What happened after that was fear. His actions are fear-driven. Where he is, we don’t know, but I feel he is a mortally scared man.
(My only other version was that maybe he planned to kill her, and then himself, and chickened out, but somehow, I don’t feel it was the case. For this, people are usually more sad to start with).