I think that Nathan is missing because he would be able to identify the perp, and the person responsible was not going to take that chance.
In order to understand what happened, I think we need to envision how this would have played out if Nathan had not been there. That is, if the parents had vanished right after they closed the office and liquidated the house, what would people assume if they simply vanished?
I agree with you, otto.
If Nathan had not been there, I believe this case would have gotten hardly more attention than the Calgary couple who were killed recently in their home (originally thought to be carbon monoxide poisoning). So, minimal attention. Adults who go missing (especially with the moving plans, estate sale, house sale) just do not whip up the media and public focus like this case has.
If Nathan was not involved, my personal opinion from reading the news would have been that the adults took off. Knowing about the blood at the scene would have made me think it was a domestic incident and that one of either AL or KL was on the run. And then the case may have faded from headlines...
I think the perp was counting on this. I still believe that Nathan's presence was a surprise and that his being there forced the perp to change his original plans somehow. IMO.