This is all speculation, of course, but to me Brian Laundrie is not behaving like someone who killed his long-time girlfriend/fiancé.
If that were the case I would expect him to try to hide, maybe even try to leave the country. Or -- if he did kill her, he seems like more of a candidate for murder/suicide.
I would not expect him to drive her van more than 2,000 miles back to his own home where everybody knows where to look for him.
To me, his behavior seems more like someone who either left his fiancé or got left by her.
But as I said, we are all just speculating.