When Aranea first meet Vriska, Vriska called her "Mindfang-look-a-like". AT first, Aranea didn't know who Mindfang was, but after asking around the post-scratch trolls, she learns who Mindfang is. Mindfang was this strong, powerful ancestor who was a skilled pirate. Aranea was a book-lover, knowledge seeking Seer of Light, nowhere near what kind of Thief Mindfang was.
As Aranea spent more and more time with Vriska, she kind of lost sight of who she was. She didn't enjoy pirates, blundering for treasures, or mind controlling her friends to face a death they couldn't prevent in anyway. That was all Vriska. Aranea learned about her other self more and more, and she tried to become the mighty Marquise Spinneret Mindfang, forgetting about Aranea Serket.
It all got to her head over dream bubble time. Her mind finally snapped. She was so focused on being Mindfang that she too far. The plan was quite beautifully put together, yes, but she focused so much on one part of her, not the other part. If you look really closely though, you can see that something else though.
Aranea Serket just wanted to be a what Vriska saw her as. Aranea want to impress her dancestor. She just wanted to be what the most important person to her saw he as, a grand pirate full of luck and fortune, not a knowledge healer of Light.