I am quite sure that people would react quite a lot like him if they lose a family member. Also lets not forget that, like Theordric already said, they have quite the long life thus losing someone hurts even more. We also cant forget that the dragons and humans made peace with each other and yet it was the humans that betrayed the dragon. I would see him different if the dragons did the first step but even Nidhogg, who never trusted the humans did not attack until they were betrayed by having their sister killed.
Also I am not sure that he knew that Nidhogg would keep this war going for so long. He may have thought that his brother will simply kill them all and be done with it (which is not really the right way but its understandable). He might not have foreseen that this would go over so long thus maybe over the time he came to the realization that he made a grave mistake. Yet still being sad over his sisters death and at the same time maybe even feeling guilty for giving his brother his eye he could have gone in a even deeper depression thus rendering him into doing nothing at all.
In the end I find both dragons to be quite tragic even if their actions, especially Nidhoggs are not morally correct.
The only dragon that I would really like to have a word with is their Father..he is really not the best father in the world. His direct children are killed of, consumed by grief or rage and he does nothing. Yes he is the keeper of the lake but really I cant accept that he would not have found some way to talk to them. I mean he could create a mini-dragon out of his corpse.
