If you could kill off one of the scions...

[Tenkuu]

I think the story made it clear that Nidhogg's was beyond reasoning, and his vengeance was insatiable, even by dragon standards.

I don't remember whether this was mentioned, but I'm beginning to think Nidhogg must have been the one to discover Ratatoskr's body. That would certainly explain a lot of his rage.

Even Nidhogg's own father pretty much deems him a lost cause that has gone completely off the deep end.

This might be true now, but if you remember Keeper's Lake, Midgarsormr had every intention upon awakening of joining the chorus. Later on as he continues to tag along with you, he mentions several times that they (the dragons) can't be stopped and that a dragon's memory works very differently from a human's, and that they will never forget what happened.