Based on BRPD stating Nathan's death is not currently being investigated as a homicide, I think the most likely scenario is that he overdosed and the person(s) he was with panicked and dumped the body.
The small transactions/checking the balance at ATMs near Happy's from 1-2am make sense if he was looking to score drugs -- if he had a history of addiction (at this point we don't know if he did or not), he may have been worried that his wife would become suspicious if she saw he made a large cash withdrawal late at night. He may have also purposely abandoned his phone on the street if he was afraid of his family tracking his location.
Since he was in a different city, he probably would not have had preexisting 'connections' he could tap into to score, so it seems he would've needed to ask someone for help. It was previously reported that Nathan asked the homeless man where he could find an ATM, so maybe he approached someone else and asked where he could score. Hopefully if that is the case, that person comes forward and speaks to police.
ETA: Another poster mentioned that Unfiltered with Kiran reported that Nathan did have a history of substance abuse, but had been sober for a long time. His belongings being abandoned (wallet, phone) coupled with his long-standing sobriety (prior to that night) makes me wonder if he may have overdosed intentionally.
