Lu Li was not a crazy man, but he had developed a crazy plan at this moment!
He must join the Leng Family, or the venom would show effect and kill him. Now that Leng Wuxue had convinced that Lu Li was Blood Emperor's apprentice, then it had become an established fact.
If so, why wouldn't he go ahead with it? As the Blood Emperor's apprentice, he must not be cowardly, or he could risk his entire endeavor by inviting doubts and suspicion.
Leng Wuxin had told Lu Li to join the External Clan of the Leng Family and to win over Leng Family's trust in baby steps so that he could find a chance to get to Leng Wushang and kill him. She even said… she could afford to wait for decades!
But that was time Lu Li did not have. He had chosen an orthodox path. He made a crazy decision. He would go to the Leng Family openly and approach Leng Wushang in the same manner.