.Why would her own father kill her? Killing your only child because she might not be as strong as you want her to be, seems rather pointless as then there's no heir at all.
The last time Koshiro was shown, he was still back at Zoro's home village in East Blue and following the chronology of the series, that was barely two weeks ago. How could he get so fast in the New World? I did however always had the believe that Koshiro was far stronger than he appears and the most ideal situation to elaborate on that would probably be the Wano country arc.
And you think that the marine battleships are freely accessible by anyone? That's the same thing as saying that the navy battleships of the American marines act like public civilian transportation. Not to mention if just those battleships would make everything that easy, the Grand Line and especially the New World wouldn't be considered such a dangerous place.
maybe he wanted zoro to inherit the wado ichimonji blade since he was a boy but he probably wished he had a son but zoro seemed like a prideful man so he just passed on the sword but not to her
That's still not a reason to kill her. There isn't a law as far as we know that forces him to give his own sword to his child, he could have given it to Zoro whenever he wanted. It would be weird that he is willing to kill her to give Zoro the sword, but not making her angry for him giving the sword to Zoro. He could even have just waited till the day Zoro beat Kuina. He could then give it under the statement of that he is the strongest person in the Dojo now and Kuina could only agree to that. Also following the cliché story of a dojo master that only has a daughter, he would have wanted to have Zoro, his strongest pupil, marry his only daughter and thus both of them succeeding the dojo.
In Samurai Traditions u hear of a man passing down a blade to a man but maybe Koshiro thought women were weak and he probably didn't want his daughter carrying the name of a famous sword and there was this picture in the manga where u see the cloth covering Kuina head maybe he sliced her in the head so maybe koshiro just thought in his head it probably would be better if zoro had the sword
Or it could just be there because Kuina hit her head badly when she fell down.
Again why would Koshiro kill his one and only daughter just because he wants Zoro to have a sword that he himself first gave to his daughter?
This is a good possibilty. Good theory bro! It could be followed by Koshiro lying to Kuina about her name and that the sword Zoro has was stolen by some bad guy which sparked her interest in gathering named swords but her father wont let her go and train again because he knew she has no potential to go further and instead turned her over to the marines.Kuina did fall down the stairs but she didn't die. She suffered retrograde amnesia. Her father took this as an opportunity to have his daughter fulfill her dreams and sent her to the marines with the alias tashigi. He freed her from the burden of inheriting his dojo and the prejudice she'd suffer and gave her a fresh start. Or at least that's what I like to think
