Personally I never really blamed Estinien for succumbing to Nidhogg's influence. He did a pretty great job at resisting for a very long time and he was - quite rightly - focused entirely on his push to protect Ishgard. Ironically it was only when he let out a sigh of relief that he ended up falling prey to Nidhogg. I do hope that Estinien himself plays a huge part in defeating Nidhogg for good, though. I don't want to see the WoL 'steal' a kill that should, quite rightly, ultimately go to the Ishgardians.