I understand that we've never seen a protagonist kill a villain and it might seem too hard for some viewers, but I think the context can soften that impact. I'll give the example of fullmetal, hence the spoiler tag.
When Mustang attacks Envy incessantly, we find revenge pleasurable because we sympathize with the character. We mourn Hughes' death, we were sad for his daughter (coincidentally the same happens with Otoko), we were sad for Mustang. Revenge is a human instinct, as morally repugnant as it is, we like to feel that feeling.
Luffy is the personification of the good person, but Zoro is not, on the contrary, we have seen throughout the story that Zoro is the opposite. He was the one who resisted Robin the most and only came to trust her after Enies Lobby.
He made it clear against Monet that if necessary he would have no problem killing someone much weaker. I know you disagree, but he is introduced as the man who killed someone else's pet, killed Mr. 7, and we don't know whether the Wano magistrate ended up dying or not, although what is important for our discussion is the scene of the attack.
I have disagreements on the first 3 points.
(1-3) I don't think there would be a problem with the power level, he left a scar on Kaido before he received his ACOC upgrade, his Hiryu Kaen without COC was a cause for concern for both Yonko. If Zoro still forges a black blade as many believe, he can gain even more power, enough to execute Kaido.
(2) I agree that Luffy always defeats the leader, I never expected anything different, in fact for a long time I said that Luffy would defeat Kaido alone and I was told that I was completely stupid to believe that Luffy would already defeat the strongest creature because future fights would make no sense. What I believe is in execution, after Luffy defeats Kaido Zoro executes him, as said by Ashura Douji in chapter 993
Very interesting this point of loyalty, I had never seen it like that. Like you, I'm sure Zoro wouldn't turn against Luffy, with the exception of the hypothesis mentioned in Chapter 6, but it remains to be seen to what extent it will be considered an order that Kaido stays alive.
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