Going away from Kaido by not finishing him is already a decision. And Luffy is there as part of Luffys crew, he wont fall against that decision. Even if Luffy wouldnt stop him maybe, but zoro well knows that everything he does there is under the flag of Luffy and in the name of Luffy.
While the citizens killing Kaido is nothing of Luffys concern. They were allies, but nothing more. If Kid kills Kaido, he may dont care, if Law does it, he may dont care too, if the Scabbards do, he wont be involved, but Zoro is his crew and he is responsible for every of their action.