I think that's what pisses me off the most about Hraesvelgr; he has just as much blood on his hands, he's indirectly responsible for everything Nidhogg does after giving him his eye, and he holds just as much hate as Nidhogg... he just doesn't act on it because of Shiva's influence, and absolves himself of responsibility by saying "time is different for dragons" and "your ancestors killed my sister and drove my brother insane, so shut up!"
Estinien says that even though Hraesvelgr never struck a blow, he got his vengeance just the same as Nidhogg, and he's entirely right. Nidhogg was his revenge. Hraesvelgr may have saved Nidhogg out of guilt and pity, but he knew exactly what that would mean in the long term.