The only person New Moon Hall was trying to kill was Maid Lotus. As for Dragon King, they regarded him as an obstructor, whom they would rather avoid than confront head-on.
They set up a series of sophisticated traps to entice Maid Lotus.
The trap began at Gulping Wind Gorge. Because it had been recognized in advance, the "scattering plan" was unsuccessful; therefore, they started to carry out their backup plans.
The traitors were fully committed to their attacks. For them, it was all or nothing. At the courier station, they used incurable poison. At the docks, they incited the crowds to attack Dragon King. They had even poured poison around the capital.
They had succeeded. Gu Shenwei and Maid Lotus had indeed been confused by such madness at the very last minute. They believed that Guan Shang must be the last traitor and had no way to escape.