When has Luffy ever said this? Luffy isn't batman, he is a pirate. His idol, shanks, had someone murdered before his eyes in chapter 1. Anyways even zoan fruits have their limits.
A lot of this is just zoro fan headcanon. Zoro fans just exaggerating the importance of their favorite character.
Fact is, Luffy is Kaido's greatest rival, not Zoro. Kaido is a fake joyboy, Luffy is the real joyboy. Being joyboy is tied to being the strongest and creating great change in the world. At least that's what King's flashback hinted.