...Is what I'd like to say, but with the Celestial Dragons being involved, there's no knowing what they could be holding over Kizaru's head. Is it possible he's doing their bidding just because he's a coward? Yes. Is it possible they have a hostage they control Kizaru with? Also yes. Is it possible that there's some complicated explanation about the greater good in Kizaru's head that stops him from doing what he considers the right thing? Yes again, but since it's Kizaru, who doesn't exactly seem like a complicated person, I'd say that one's unlikely.