20 years trying to find the man who would kill him, and just when Kaido thinks Luffy is the guy, the same thing that happened to Oden, getting screwed by an outside force, happens to ruin his 1v1.
The fact that Kaido looks this sad, because he WANTED Luffy to kill him in that moment, in a glorious 1v1 battle, is so heartbreaking. Kaido is a monster, but he's clearly less heartless than he's made out to be.