Whatever name he goes by - Vader or Anakin - they are both the same person. The names are just symbols. Metaphors. "Anakin" is the part of him that is kind, forthright, generous and loyal. That part of him was always within him. "Vader" is the part of him that is fearful, selfish, possessive, and wrathful. That part of him was always within him too.
Basically it's the Two Wolves fable, and Vader was the wolf he fed the most. The names don't really matter, so best not to take it too literally and think that this is anything like a split personality.
So are Vader and Anakin really the same person? Of course they are, and at the same time; of course they are not.
Yeah, it always feels like the Jedi stick to such a strict code because the Dark Side will literally warp their mind like a bad drug if they let it in.
The 'Dark Disciple' novel gets into this a bit, but yes. It's essentially a snare. A trap. It promises everything a person wants, but the deeper they go, the more it consumes them.
Where the Jedi failed isn't in the code itself, it was in following it so dogmatically without really considering why it is what it is. Taking for granted that:
"you do this, then you're light side and therefore good, and being good then everything you do is by definition of the light side."
The Nightsisters deliberately ride the edge of it, knowing full well it's dangers, it's deceptive and seductive nature. I think they mostly get away with is because of the very nature of Dathomir; it's steeped in the dark side to such an extent that it's become part of the natural balance of the place, dark and light both. This seems to line up with the fact that once they leave their homeworld, Dathomirians and especially the Nightsisters do seem to have a tendency to be consumed by it, or at the very least struggle to find equilibrium. While on the flip side, outsiders find the place deeply unsettling.
