Shanks and Kaidou had a fight and it ended tie with no casualties. And as I assumed before. Kaidou might want to be killed by Edward, but Shanks stopped Kaidou, perhaps due to this balance thing in the world about admirals, Shichibukai and Yonkou. Perhaps Shanks was holding back of killing Kaidou back that time. Otherwise, Shanks could kill Kaidou if he wanted. However, Kaidou is at his prime, and whoever gonna defeat Kaidou, will power up pretty much.