I would argue that one main difference between Estinien and Nidhogg's vengeance is how all consuming it was and how it was directed. In Estinien's case though he had fallen under the influence of his rancor in the DRG questline, he still is notable that A) his primary focus of his revenge is the individual responsible and B) he is open to restraint for the sake of his duty. He didn't believe peace was a likely outcome but he still allowed the attempt. He didn't take any action to try and undermine our effort. Estinien's vengeance was a defining and controlling aspect of who he was but it wasn't all he was. Nidhogg on the other hand was so completely consumed by his rage and thirst for vengeance there really wasn't much else left. This was compounded by the fact that the actual people behind the crime were all long dead. Nidhogg was simply taking out his rage on whatever target fit as an outlet. This shows particularly when he and his brood attack other dragons and their servants such as in the FATEs in the Churning Mists.