who did kakashi kill to get the MS?
and why does his vision not fade in his eye after he has been using it?
-Obito and Kakashi saw Rin die so their Sharingan matured into the Mangekyo.
-Obito's doesn't go blind due to Senju DNA.
-Kakashi's Mangekyo is in fact going blind. That's how he knew that MS deteriorates the eye.
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The question is how blind is it?
noce point but its possible kakashi did some research on it
How? This isn't something that you just look up in a scroll. The only way for him to find out MS deteriorates the eye is for him to experience it first hand or if someone told him. Itachi never mentioned MS's blindness drawback and no one else had MS to tell Kakashi of it. He doesn't know where the Naka Shrine is so he can't read the tablet (Itachi said it was a secret location) Thus he had to have experienced it firsthand.
actually im pretty sure thats how people know about EMS madara included
noce point but its possible kakashi did some research on it
Madara knows where the tablet is so he can read it. Kakashi doesn't, nor does he have anyone to tell him.