Why is Nurse Ratched considered a villain?

Because while she is professional, she also doesn't seem to care about the patients except in a superficial way. She also reveals her cruelty when she finds out Billy Bibbit has had sex, a normal desire, by threatening to tell his mother and embarrass him. She doesn't care about the patients finding happiness or bettering their lives...she simply wants to go through the motions and maintain her authority over them.