Choosing an outcome and searching for evidence afterwards is an illogical and unscientific theory.
It's only illogical if the outcome isn't suggested in the manga.
Which, for ZKK, it can definitely be argued that it is suggested in the manga.
If you said "Zoro defeats Blackbeard (while Luffy naps) because [insert 'evidence' here]".
Where evidence is junk like "Blackbeard is related to the number 3 and Zoro uses 3 swords, Aha! Zoro defeats BB confirmed!"
Then, that is totally different. That's an outcome that wasn't suggested in the manga and that you came up with yourself...