He was from the bandit group, he was saved by Blackdragon so he was loyal to the group. However the Watersnake did not like his rise in the group.
He was invited by Leng Qian by orders from his master Ouyang Shuo, to enter the army of Shanhai City.
He rejected Blackdragon for his loyalty. But after Blackdragon was killed by Watersnake. Watersnake was soon killed by General Shi who had entered with Shanhai's army.
For the second time now Ouyang Shuo personally invited him to be his general, he soon accepted.