The One Piece theory explains the most likely reason behind Kuina's death.

Fans of Roronoa Zoro have known that he has dreamed of becoming the strongest swordsman in the One Piece world since his childhood. Although his past has not been explicitly shown in the series, fans have a pretty good knowledge of his lineage and how being a swordsman fits with his personality.

While Zoro's dream has been part of his ambition since childhood, he only became extremely serious about it after his rival and friend Shimotsuki Kuina, daughter of the dojo's master, Koushirou, died. Kuina and Zoro trained together in the dojo, and she was one of the few characters who could easily beat Zoro.

Although Kuina's death in the One Piece story comes from her falling down the stairs, fans have never believed this story. Thus, there have been many theories about her demise, and the most likely of all these theories makes Zoro's past even more disturbing than it already is.