The death of the old friend whom Han Wu had tricked into a duel and then ambushed was a thorn in Blood God's heart. In his mind, the priority of avenging his old friend and exterminating the entire Han Wu's lineage was even higher than seeking revenge on the Dragon King.
Now that a descendant of Han Wu had entered the Minor Spiritual Realm, he wanted to extract some kind of repayment for his old friend. The Human Race prided themselves on being able to deceive others, didn't they? He wanted to weave a lie that would let this person taste the pain his old friend had endured.
In the Demon King's view, Blood God's plan was full of flaws. How could the crafty Human Race be tricked by the Demon Race? If the human was a coward, he wouldn't dare to approach the underground palace, let alone stay for a month.