The first thing one assumes is, he isn't too bright. He is not a forward planner, he is a reactor, a knee jerker, a person who lives in the moment, a grenade thrower, who has so far escaped cleaning up his own mess...
The second thing is, ... and this one is the real reason, he wanted to go home to mum. Now.. I am not saying that Mum rules that household, it could be home to mum and dad, but generally, manbabies want mum more than anything. Gabby mothered him, I suspect.
There really is no rational reason why he would risk that long drive in a stolen car, with a battered body of his girlfriend lying around in a well used national park, unless he was convinced that he had a soft landing and protection and assistance and emotional acceptance and financial and practical help to clean up this mess. Absolutely no reason that stands any scrutiny.
And he was right. There was a lawyer waiting, the camper packed up and ready to go, plans afoot, everyone one the same page about no-speakies to the Petito's...