Yes and no. The English version portrays him as outright hostile, but the Japanese dialogue paints him as far more ambiguous. He never says he's going to join Nidhogg, just that he's not going to stop him either. It's rather similar in the English dialogue, though he does say that he's "[rising] to join in the chorus" and the Japanese Midgardsormr says "Do not think I spare your life for no reason."
Reading into ambiguous characters like Middy takes a step back and objective viewing, but he was never outright hostile - the "fight" against him is, like Ramuh, just a test to prove our worth, and he did nothing to actually topple Ishgard in the millennium-long Dragonsong War. He's very much an observer rather than someone on the stage - the only thing he actually does is strip you of the Blessing of Light and then spirit you out of the exploding Singularity Reactor at the end of 2.5 and 3.0, respectively.