Why did the Jade Emperor send Wei Zhi to chop off the head of the Dragon King?

Because King Jinghe changed the hours of rain privately, which broke the dogma.

King Jinghe was very angry because Yuan Shoucheng could calculate the location of Jinghe Aquarium. He turned to Untitled Scholar, went to Chang 'an to find trouble with Yuan Shoucheng, asked him to calculate the time when it would rain tomorrow, and left a message saying that if the calculation was wrong, he would be driven out of the city.

As a result, heaven suddenly decreed that the number of rainfall points in King Jinghe tomorrow would be exactly the same as that calculated by Yuan Shoucheng. King Jinghe refused to admit defeat, changed the hours of rain privately, broke the dogma, and the result was known by heaven.

Yuan Shoucheng asked him to ask Li Shimin for help, and Li Shimin promised to save him, but he was finally beheaded by Wei Zhi, the heavenly teacher on earth, in his dream. After his death, his soul has been rectifying Li Shimin. Finally, he took Li Shimin's soul to fight against the nether world. The nether world favors Li Shimin and releases him to the sun. King Jinghe's soul was forced into reincarnation by the ghost.

Extended data

King Jinghe is just a victim of a political transaction and an insignificant pawn in the eyes of the Buddhist circles in heaven. Obviously, Emperor Taizong was just an executor. In the whole process, the nether world, heaven and Buddhism can play with him at will, and he has no choice.

It seems to be a journey to the west, but in fact it just follows Guanyin's instructions. Emperor Taizong and King Jinghe are just pawns behind the scenes, and they have no right to speak in the world of immortals and buddhas.

References:

Baidu Encyclopedia-Jinghe Wang Long