Hraesvlger is considered by the story in my two views of it as negatively complacent and is ultimately an accomplice to his brother's acts. His dad has to effectively come in and go "You got to get out of your room and stop listening to emo music, a new world awaits now." to get him to stop sulking and fucking do something.
While people like Aymeric are too sympathetic to really argue with him, Aymeric has shown times to be not very good at actually arguing when you hit the right emotional notes with him (his father especially). Alphinaud, Estinien, Ysayle, and Midgardsormr make varying attempts to show him that lingering on this is a mistake and he is in a way no better than Nidhogg especially considering the aid he provides via keeping Nidhogg alive. Nidhogg causes torment to Ishgard for the sake of sick gratification against people, who at this point have done nothing to him that he himself didn't instigate. Hrasvlger on the other hand is paralyzed by indecision due to a mixture of guilt and depression, so he feigns neutrality because he lacks the will to make a decision. He is a fence sitter on a grand scale.
Nidhogg is considered wrong by basically everyone who's sane. What everyone hates about the government isn't what they did a trillion years ago per say, we all mostly all generally agree that being punished for this shit that happened before literally anyone alive today was born is wrong. Why people hate the Catholics for is that the only reason it continues is for the sake of vengeance and to maintain a status quo that the pope benefits from. Thordan lacks the will to bring about a new era despite knowing the war he subjects his people to is wrong, so he's finds a work around to his hangs up with complete arrogant reckless abandon by becoming the primal King Thordan so he can just smite the dragons away.