Ling Han decided that they would return after Shui Yanyu successfully endured her heavenly tribulation.
They had already killed enough pirates, and the pirates had all become afraid of their guerilla tactics anyway. Now, all of the pirate ships would have at least one Sun Moon Tier elite guarding them. This deterred Ling Han from attacking them as the gulf in power between those at the Sun Moon Tier and Mountain River Tier was far too great. Just small mistake could get him killed.
Even the Black Tower and Cloud Piercing Shuttle wouldn't be able to help him much. Moreover, how could he expect fortunate coincidences—like Xi Cheng stabbing his Absolute Beginning Stone—to happen every time?