Today I was reading a book by Jonathan Clements called “A Brief History of the Samurai” (Yeah I know Zoro is not a samurai) and at one point the book talked about swords referred to as “Muramasa swords”. These swords were known as cursed swords because “their creator was half-mad, and the weapons he made could impel their wielders into murderous rages (in Whiskey Peak Sandai Kitetsu forced Zoro to cut at times he didn’t want to).
One story tells of a young samurai, Gentaro, up in Edo as part of his obligatory domain service, who sees a Muramasa hanging among the other swords on a dealer’s rack” (Zoro found the sword in a barrel of swords that Ippon-Matsu was selling for 50,000 berries each) :
“Trembling, he withdrew it from his scabbard, and he forgot to breathe. The sword had hidden depths, like morning mist welling up within the metal. Light danced off the blade in the colours of the rainbow…Just by holding it in his hand, he could tell it was a masterpiece… (Just by holding Kitetsu in his hand, Zoro could tell it was cursed)”
“The samurai is even happier when the dealer offers to sell him in the blade for a fraction of its true value”, although he will not say why…(Tashigi revealed that Kitetsu was worth much more than Ippon-Matsu was selling it for. The sword shop owner however, did say why he was reluctant to sell the cursed sword).
The inspiration behind cursed swords could possibly reinforce the theory of Zoro’s left eye, that it was cursed and drove Zoro into a blind killing rage (it is said that the eyes are the windows to one soul, and Asura could be the manifestation of Zoro’s soul).
*Theory by PirateKing Monkey D Luffy