Actually, there was a hint of regret in Sun Wukong's heart because, among the people of the Netherworld World, the existence of an undercover was something he kept saying he didn't believe and didn't want to believe. However, subconsciously, he had already accepted it and even had a vague feeling of who that person might be. Precisely because of this, he was even more unwilling to believe it, preferring to deceive himself and tell himself it wasn't true but merely his own misconception.