Could have been someone needing a quick drug fix and knew she was alone I suppose, preying on the elderly. Preying on anyone for that matter. This world.......
The drug angle is one I have thought was a real possibility.
Example: A local worker, worked for NG at her home( roof for instance). Unfortunately,
The perp opened his trunk to get a tool, right as NG walked past, she saw drugs( 2 bricks cocaine, he thought he had covered it better!). He kicked himself for not stopping at home before work. His mistake.
He quickly closed the trunk, just as she said, “Hey, is that drugs in your trunk, it sure looks like it!” He laughed it off, “ No, just supplies for another household repair job.” He felt satisfied that she believed him, until later that night.
Later, when the worker went home, he rethought it all and he panicked! His paranoia took over, He thought of his drug boss.
Plus, he had gotten used to the extra cash, he wanted to keep it going.
He thought NG might turn him in, he can’t have that. NG needed to be silenced, gone. He had never thought of this before, and hatched a plan to take out NG very sorry….deposit body in the desert. Just a simple plan, to make sure NG was gone forever, could not bust him. He carried on with his life, as if nothing had happened, problem solved.

ugh.
Something “simple” like this example, close to home, motive is simply that NG needed to be gone, to avoid talking, exposing the perp’s secret( local drug). Ramped up as he was paranoid from cocaine use. Just another idea.
All JMO