Why is Qin a keeper?

In the novel The Journey to the West, it is said that the old dragon in Jinghe near Chang 'an made a bet with the fortune teller and broke the dogma. The Jade Emperor sent Wei Zhi to supervise and beheaded the old dragon at 3 noon. The day before, Lao Long asked Tang Taizong to intercede for him, and Tang Taizong readily agreed. The next day, Emperor Taizong announced that Wei Zhi had entered the DPRK and left Wei Zhi to accompany him to play Go. Unexpectedly, at three o'clock at noon, Wei Zhi dozed off and dreamed of beheading the old dragon. Lao Long resented Emperor Taizong's treachery, pestered him, and made trouble in the palace every day, which made the six gods of Emperor Taizong uneasy. Knowing that the emperor was afraid, Wei Zhi sent two generals and Weichi Gong to guard the palace gate. Sure enough, the old dragon dared not make trouble. Emperor Taizong remembered their hard work of guarding the door at night, so he asked the painter to paint their portraits and stick them at the palace gate. The result still worked. As a result, this move began to spread among the people, and Weichi Gong became a keeper.