Chapters ago.
Defeating =/= Killing
Even defeating is not confirmed. What if Luffy and Yamato won't manage to strike down Kaido by themselves for instance and Zoro will need to also help them defeat Kaido.
ZKK is an entirely different matter;
- Luffy and Yamato will presumably not kill Kaido for obvious reasons
- Zoro's expressed will
- Kaido's expressed feelings about death = perfection and his attempts to test himself
Just a few arguments.