That's more of a grey area, I'd say. Isabelle has done some very bad things, but the worst of them have been under Alec's direct instruction and supervision, with threat of violence if she doesn't follow through. Other bad things she's done without him making her do them are of course more clearly wrong, but then there's the environment she's in to consider. I don't have a clear-cut answer myself, and I would say that if Isabelle is a bad person, she's one with significant hope for redemption.