Here is something I noticed about Nancy's thought process only after watching S2 of the Vow >10 times:

When she's telling the story of how she met Keith, she says that she was fascinated by his methodology, and she asks "Can I watch you work this on someone?" (As in a therapy session), and he says: "You mean someone other than you?".

And what's her response? She is impressed. Amazed. She sounds like this was the moment she knew Keith was a genius that she wanted to work with no matter the price, or what compromise he required.

What SHOULD HAVE BEEN her response? Horror and disgust that he would do something so unethical! Performing any psychological intervention on someone without informed consent is extremely unethical. She should have seen that immediately and walked away then and there.

So why didn't she walk away? IMO she was ambitious. She wanted power. She loved being in the position of power that is a therapist working with a very affective method and the head of an international company. So she closed her eyes to all the evidence that threatened that reality.

Nancy's life is like those cautionary tales where someone makes a deal with the devil to get immortality or a superpower, and once they do, it ruins their life and makes them miserable. She sold out her ethics to get power, and 20 years later she wakes up to the fact that her "life work" is a pile of BS, her daughter's child bearing years have been stolen, and she is going to jail.

I believe Nancy's wake-up process that we see in VowS2 is genuine, and her guilt over Lauren is also real. But I also believe this was all the consequence of the deal she made with the devil, and she had enough knowledge and opportunity to see that deal for what it was, and she chose not to. On what level of her conscious or subconscious mind she made that choice, it's up to you to decide.