Do you want Kaido to kill them there? If Law escapes, Kaido will search for him and may find out about Kin'emon. He and Luffy being prisoners and have there crew seen as death, is the best outcome to keep the plan going. Law has enough foresight to see that.People would lose all respect for Law if he ever did some nonsense like this lol
Also it puts the three best Nova-Capatins in one spot and them working together is foreshadowed all the way back at Sabaody Archipelago.