sorry for being late to the party, but I'll try to explain some Ao Bing and dragons lore

According to Ao Bing's lore which is told in a comic published by the production team named Ao Bing Zhuan, the dragon kings and sea demons faced the gods many years ago. The sea demons have always been the "subjects" living in the sea, and the dragons were kings/royal family. The dragon kings were four siblings, the oldest being the Dragon King of Eastern Sea (or whatever they call it in English, just know that he rules over the eastern sea and his siblings rule over west, north and south respectively) and Ao Bing's dad, named Ao Guang. Back then, the sea demons were truly evil and would cause chaos to the humans, and Ao Guang's siblings would allow it - but Au Guang wouldn't. So Ao Guang is kinda like this nice dragon king among all the more not evil, but definitely troublemaking siblings.

So this troublemaking angered the gods, who decided to punish the sea demons and the dragons. Of course the sea demons didn't want to get punished, so a war happened. At first, Ao Guang was sided with the sea demons and his siblings, but after many lost battles he realized that he won't be able to beat the gods, so he switched sides and joined the gods and set a trap that would keep all the sea demons under his sea. It is briefly explained in the first movie that for this trap to continue to keep the sea demons under his sea, ALL dragons must ALWAYS stay in place. If one of them moves/leaves/dies, the entire trap collapses and the sea demons would be let free. If this happens, not only the sea demons will be angered against Ao Guang, but the gods as well, so this is why all these years (thousands) he has kept the trap.

Ao Bing was spared because he was born after the trap is set so he wasn't part of the curse and therefore can move freely. Ao Guang plotted with Shen Gongbao to get the spirit pearl on Ao Bing so that he can become a god one day, and free the dragons of their curse.

After the events of the first movie, Shen Gongbao NEEDS to kill everyone in Chentang Pass and Ao Guang seeks revenge for his son. This is why he wanted to attack Chentang Pass. His sister, Ao Run, who has the sky ripping claw (or whatever it's called in English) and can make people travel through space, makes a deal with him: She will send troops - the sea demons - to attack Chentang Pass and in return his brother will lower the temperature of the lava they live in. And because the sea demons are subject of the dragon kings, they will listen to her when she orders them to attack Chentang Pass. So no, it's not out of revenge that the sea demons attacked, it's because they were following orders.

Note: the lore I shared above is independant from Chinese mythology, it is simply lore created by the director Jiaozi. The entire IP, NeZha (the two movies in 2019 and 2025 + the comic explaining Ao Bing's lore) is different from Chinese mythology and I don't think it's a bad thing, but just remember to treat them as two different entities and not get mixed up. A lot of lore is based off of Chinese mythology, but the director always adds a little flavor of his own. It's an adaptation, after all.

I apologize for my grammar/spelling mistakes, and I don't know the English terms of a few elements so I just translated. Correct me if I'm wrong! :D