What I find somewhat damning is that he took her car and drove 2000+ miles.
I also don't understand (not just in this case) how someone could get so enraged at someone that they supposedly love, that they kill them. It's so much easier to turn around and say "See ya!".
Love, especially in the physical sense is one of those emotions that thrives on passion.
The line separating a really hot night in the bedroom and a crime of passion is thinner than we would all like to admit. A passionate time gone wrong could lead to violence that might have been a pre-meditated murder of involuntary manslaughter.
That may have caused the incident leading to death but the actions afterwards are not indicative of Self-defense or a fight gone wrong. If this was an accident call 911 and seek help immediately.