This is still a confused and confusing story. It might be better to wait a while before coming to any conclusions.
I dunno. According to news reports:
Eye witnesses reported a dark blue Tacoma pick up leaving the scene with a Navy insignia on its lisence plate. His mother was spotted in a truck matching that description at a rest stop in I-495 in Chelmsford, MA. When State Police approached, she was reading a bible. When they asked her if she would come to the station (actually a “barracks”) to speak to them she agreed. While there she confessed to killing him with an overdose of cough syrup.
There doesn’t seem to be much room for doubt about anything except her mental state.