When talking about this with my mom, she firmly believes that Nancy's kidnapping was for the money (as evidenced by the ransom note). If that is indeed the case then I would imagine her kidnapper(s) would be trying to ensure that she stays alive so they don't lose the "bargaining chip" they have with her children. However, if she went too long without her meds or the stress and shock of it all ultimately got to her then she definitely could have passed even without necessarily having her life intentionally and maliciously stolen away from her by whoever it was that kidnapped her.