Kizaru has been dragging his feet as much as possible and literally playing games with a stalling Luffy. However since the title "Luffy vs Kizaru" has already been used, I think his serious demeanor is supposed to contrast with the next G5.
At which point joyboy will do joyboy things and make Kizaru laugh. I consider his betrayal a foregone conclusion, but Kizaru could easily be part of the New Kuma Pirates instead of joining.