Where was that said?
Uh...isn't that the difference between LS an DS? Perception and views of evil and all that? She was acting on her own free will, Malek wasn't controlling her mind. She was a Dark sider, just as Vader was. It doesn't mean they are past redemption tho, or have no good in them.
You forget that Revan can just as easily keep her on the DS, she sure seems to buy into Maleks "lies" then.