Now that Luffy, Kid, and Zoro are presumably heading for Kaido, and Apoo's still active, it is time for Killer to step in and finish off Apoo? Killer was the one hurt the most by Apoo. Killer was the one who invited Apoo to join an alliance with Kid, and Killer was the one who ended up becoming a Pleasure because of Apoo's betrayal. Killer has shown knowledge about how Apoo's powers work, and his mask may help with blocking out the sound, so it would make sense for him to fight Apoo
