but then why didn't he unlock it before, why did it take him so many years.You don't have to kill, you just have to witness the death of your closest friend, which in Kakashi's case would be Obito or Rin.
It's not an instantaneous thing, especially if you don't know about it, which he didn't. Additionally, there's no way for Kakashi to have the prowess with his sharingan that a natural Uchiha would, especially with only one eye.but then why didn't he unlock it before, why did it take him so many years.
Uhh.. yeah he did. He killed Itachi, thats when he unlocked his mangekyo. He was the closest person to him at the time, whether he saw him as good or bad.sasuke didnt kill anyone to get his MS soo idkk