Shangguan Yun proposed no assassinations between Golden Roc Fort and the Dragon King. It meant that he intended to have a final fight with the Dragon King on the lake instead of the land.
The killers of Golden Roc Fort could easily defeat Dragon King in assassinations, but they had not made use of all their strengths. Up until now, the Fort had not sent their secret weapons, the black-masked assassins with the highest kung fu skills. Shangguan Yun had an advantage over the Dragon King in regards to assassinations, so he had no reason to stop fighting.
Shangguan Yun managed to persuade many people in Xiaoyao Lake, except for his half-brother. In his eyes, Shangguan Hong was just his messager.