Just completed the quest The Rising Chorus in ARR 2.5, and "met" Midgardsormr.

I am a little confused about Middy on whether he is bad or good?

Seems the history is that he helped defend Eorzea from the Garlean attack of airships (good), but he fought me and took my crystals (bad). Now he is following me around to keep an eye on things (good)?

Also, he is planning to attack Ishgard because of something that was done in the past (bad).

Can someone kindly give a brief recap on what is going on at this stage in the story without spoiling anything? i.e. why does Middy want to attack Ishgard, why did he take my crystals, and why is he following me?