I think you said it best. As a nemesis, she's not some "embodiment" of evil. It's her values opposing RP's that makes her such, the belief that what she truly believes what she's doing is right. I used to work with an IT manager who was a) very religious but b) so unbelievably off-putting to others around him. In a discussion with my own manager, I noted that "Sam" wasn't a bad person. He just truly believes what he's doing and his way is the right way. He's driven by his values and is so incapable of seeing anything from another's perspective. But Nurse Ratched does go a bit deeper - she's malicious in the way she attacks Billy's psychological weakness: his mother. That's what makes those nemesis characters great - they expose the fundamental and psychological weaknesses of others, much like the Joker does to Batman in The Dark Knight.

[...] Great Characters: Nurse Ratched (“One Flew Over the Cuckoo’s Nest”) (gointothestory.blcklst.com) [...]