I'm not convinced Hydaelyn has any nefarious purpose. She is constantly portrayed as a positive figure. Her words in the theme songs, what she has asked of the WoL and even down to the first thing she said to us. Feel.. Think..

I honestly feel she wants us to think and act for ourselves. To learn and grow. She certainly doesn't hand us a lot of marching orders. Whatever she and Zodiark truly are, I don't think she is on the 'bad' side. If Zodiark is the true god then maybe its a good thing he isn't active. Being a true god doesn't mean being a nice or good god.

The Hydealyn/Primal thing doesn't look likely to me. Primals consume Aether, they don't hand it back out. If Hydealyn is a primal then realistically from all we have been given, we would be part of that primal as she provides Aether to the world. Also no one acts tempered towards Hydealyn, including the WoL. Being tempered means being unquestionably devoted and obsessed with the Primal. You exist for that primal. That doesn't seem to be the case for people and Hydealyn. If the WoL was tempered it wouldn't matter if Hydealyn was bad because they would have no free will anyway.