A recurring theme in Naruto is that power corrupts. There was a reason why the fruit was called a forbidden fruit, if anime has taught me anything is that when you gain a power up of such magnitude there is usually a cost. One of the cost may be a corroded mind, in order to support this we can look at the eyes that closely resemble the rinne sharningan to understand it's properties. Isn't it a bit strange that the only way to gain power from the sharingan is to go through immense turmoil? Perhaps that is the curse of the fruit, those who eat it will gain massive power but will retain a corrupted mind. A better question is, where did the shinju fruit originally stem from? Why was it that only the blood of man was able nurture the tree as stated in the tree's back story?
Everything surrounding the shinju tree seems to revolve around bloodshed so I wouldn't be surprised if the curse of a corroded mind came, the shinju tree grew from blood shed, which means the rinne sharingan grew from conflict and bloodshed, so to then would the sharingan stem from conflict and possibly bloodshed. It all seems connected, the only anomaly is the rinnegan however one needs hagoromo's power and the rinnegan stems from the shinju, present when victims are placed under IT as seen during that portion of the arc. Ultimately power corrupts which is what the shinju fruit is cursed to do, as Hagoromo stated it must have had to do with something with the fruit.
Surprisingly as hamura and hagoromo didn't seem corrupted despite their powers stemming from the shinju, well their mother kaguya but still. If the sharingan causes a chemical imbalance in the user when it's fiest awakened would that also apply to the rinne sharingan on a greater scale? If so then that would explain Kaguya's corrupted nature. I would think so, just a theory.
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