Intentional kidnapping, probably with a sexual motive. I believe the ransom demands are all fake, from opportunistic people who have no connection to the actual crime.
The kidnapper may not have known who Nancy was, beyond an elderly woman living alone.
He could live locally or work in the area in a profession or trade that would make him unnoticeable in that environment. That would have given him a way to observe her movements beforehand without alerting anyone.
I don't think she survived very long, was possibly even killed at her home. He's obviously found a way to store or dispose of her effectively, as she hasn't been found. Possibly in a freezer, since the geology makes a burial impractical.
All very much my opinion only based on available facts.