The death maniac only left a spoiler like prediction and went ahead of him, regardless of how much doubt he had in his heart. He still doesn't know where the death mania exists in what form. If she can only stay in hell, how can she know what happened to them in reality? If she is not in hell at ordinary times, how can she know what will happen in hell?
Arthur didn't even know whether the death maniac was his enemy or his companion. If it's the enemy, why does she care so much about herself and Sonia? If she is a companion, but she is also a member of the final five, their ultimate goal is to capture Yashu and others