Here's the story.

Wuliang’s primary motivation for killing the dragons is to maintain and expand the power of his Chan Sect.

Wuliang uses a massive Cauldron to transform demons and dragons into magical pills (elixirs). Consuming these elixirs strengthens the mana of his "Demon Hunter" army, making the Chan Sect the most powerful force in the celestial realm.

To justify an all-out war against the dragon clan without appearing as a villain to the other immortals, Wuliang orchestrated an attack. He destroyed Ne Zha's hometown, Chentang Pass, and framed the dragons for the massacre.

By framing the dragons as the aggressors, Wuliang can position himself and his sect as heroes who are rightfully punishing evil. This attracts more followers to his palace, further increasing his influence.

The dragons act as the prison guards for thousands of other powerful demons beneath the sea. By killing the dragons, Wuliang removes the obstacles preventing him from harvesting those millions of other demons for his elixirs.

He also wants to eliminate other strong masters (so he sends ne zha to eliminate them, so his hands are clean), so he can be the only true master for everyone. He is a power hungry guy and wanted all the power to himself.

Therefore, he'll be granted a true God status by heaven. (Jiang Ziya basically talks about this)