King Piccolo was among the many targets Yinsen needed to defeat. After all, the King Piccolo arc had been a crucial turning point in the original Dragon Ball.
That arc gradually abandoned its initial comedic martial arts style and shifted toward a passionate battle-oriented style.
Furthermore, King Piccolo's arc had originally been intended as the ending. It was hardly difficult to understand why such an important villain had become a target Yinsen had to defeat.
Yinsen was certain that, to complete his mission, he needed to defeat not the aged and weakened King Piccolo, but King Piccolo after he used the Dragon Balls to regain his youth and return to his peak strength.
"I didn't expect I'd have to start messing around this soon..."
Not only did he have to defeat him, Yinsen first had to help King Piccolo find the Dragon Balls and make his wish. This kind of messing around... was even more speechless than Vegeta later allowing Cell to absorb Android 18.
"This completely goes against my usual style."
"Yinsen, what exactly are you talking about?"
Yinsen had yet to realize just how great an impact his words had made on Master Roshi.
Could it be that Yinsen wanted to join the Demon Clan? Why would Yinsen do that? There seemed to be no reason for him to do such a thing.
Tambourine across from him also wore a baffled expression, unable to understand what this human was trying to do.
"Do you wish to pledge yourself to my master's service? I didn't expect you to have some self-awaren—"
Before he could finish, Tambourine was sent flying again by Yinsen. Had Yinsen not held back, the guy would have been beaten to death already.
"Pledge myself to King Piccolo? You sure know how to tell a joke!"