I feel another event would just plummet her to darkness anyway. She seems like the type of person to let revenge cloud her mind whether it be a minor bad experience or a traumatic one such as her traumatic experience in the novels.
While I do agree that a bad experience can indeed turn a person "evil" I don't believe it defines a person and always makes someone "bad". I believe Neferet could have chosen good and still could but chooses not to. Nobody is defined by their experiences yes they affect a person but does not define you. You chose the way you are and the way you want to be.