Hate Ryuzoku
Valgarv's hatred explained.
During the Kouma War, the golden ryuzoku were looking for allies to help them defeat the mazoku. They knew that the ancient ryuzoku possessed an incredibly powerful weapon that would turn the tide of battle in their favour so they approached the ancients and asked them to join the battle. The ancients, fearing that the use of the weapon would only cause more strife and chaos, refused the goldens in favour of staying neutral in the war.
The golden ryuzoku did not like being refused. Knowing full well how powerful the ancients were, the goldens became paranoid that the ancients' refusal to help them in the Kouma War meant that they could very well turn on them in the future. While their belief was utterly unfounded, the goldens became consumed by it. They didn't want to lose their place as the leaders of the ryuzoku and they feared for their future. As a result, the goldens decided the only solution was to kill every last ancient before they had the chance to turn on them.
Thus, the goldens began their genocidal crusade. Orders were given for the golden ryuzoku forces to move en masse to kill every single ancient ryuzoku they could find. None were spared: children and eggs were slaughtered, all because the goldens were too paranoid, selfish and power hungry. To make things worse, they created several barriers to seal the corpses of the ancients within a temple in the frozen wastelands of the north. Here, the hoped that the ancients would be forgotten.
When Filia finds out about the ancients and asks her elders about it, she's told that the ancients were a dangerous, violent race that needed to be put down to ensure the safety of all. Filia nearly believes this until she ventures to the temple and finds the truth. Disgusted with the lies, cover-ups and the crime itself, she forfeits her position as priestess.
To Valgarv, the golden ryuzoku are no better than the mazoku in that they will happily resort to genocide if they are unable to get their way. Valgarv grew up in a world that hated him simply for how he was born and he spent his entire life running and fighting for survival. He can never forgive the goldens for playing god and taking the lives of so many innocents.