This is indeed a very interesting question that I have been working around to find the answer. No one is going to be able to know the right answer to this but we can speculate or make an assumption as to how Kakashi obtained his MS.
My assumption:
I haven't seen anywhere or read that a non uchiha would be able to take his/her sharingan to the next level.
When kakashi met Itachi before getting to the cave where garaa's tailed beast was being sucked, Kakashi made a indirect approach to Itachi saying "how good is your vision?" a question similar to this. I am not sure I would have to check. Itachi immediately knew what kakashi meant by that question and he responded "kakashi you.... Impossible". This means that Itachi too believes that kakashi wouldn't have been capable of awaking the MS. Obviously Kakashi knows everything behind the secrets of the MS. He probably even went and read the tablet of the uchiha.
You can not just assume that kakashi killed his best friend. He is not this type of person. He is not going to kill his best friend for power or someone dear to him. This is just something we all know kakashi wouldn't do. And we all know that Obito was an outcast to the uchiha and the only people obito considered as friends to him were kakashi and rin and probably the fourth hokage.
There is no way kakashi awakened the MS. Someone had to awakened for him. Someone who is an uchiha. Or in fact someone named Obito.
This gives the possibility that Obito is indeed alive and that there is a possibility he killed Rin to gained MS by being manipulated by Madara (Yes, to you other people who do not believe obito is alive or even give it a chance that it's a possibility. I do and I am not going to change my mind).
Obito/tobi/mask man/madara awakened the MS before kakashi even realized that he had MS powers. He just needed to know the next level of the sharingan. This is why we see madara being able to master the MS power that kakashi has. Madara's MS power and kakashi's MS power are similar. Kakashi is just a beginner in using the MS while madara knows all about it and can use it to perfection.
We also see another a picture where madara looks deeply into kakashi when they meet. Madara is staring at him with a somewhat angry look.
It's a very long assumption/theory that I have been developing during these days. It's just one the mysteries I am trying to figure out.
Why would kishi choose not show obito's grave or corpse?
Why would kishi hide how Rin died or when and where or what happened to her?
Why is madara hiding his identity even though everyone knows he is madara?
these are the questions I am constantly asking myself. The more I think about it the more I believe that obito is alive and that he took control over obito's body.
no no no no no you had me at Obito being the outcast to his clan.
Obito wasnt an outcast. he was considerably weak though compared to the average Uchiha.
just as its not in Kakashis character to kill a close one its nowhere near in Obitos either. Obito is the one who made Kakashi realize the important of not abandoning their friends.
btw Obito's dead. crushed by rocks. and he died right after unlokcing his regular sharingan powers. Rin is the one who implanted Obitos eye in Kakashi so Rin was not killed by Obito. Rin however died some time after that whole event. Obito was under boulders buried. Madara would not need his dead corspes. Madara wouldnt know what to do with a dead corspes. if anything he IF he found Obito body somehow under a pile of boulders he would take Obito's right eye but that eye was crushed soo Obito's dead body is useless.
only reason to show Kakashi Gaiden was to show how Kakashi got his sharingan, Kakashi's growth from strictly following rules to putting his friends 1st, a lil bit of the 4ths coolness and Obito was a Uchiha version of Naruto and his only purpose was to teach Kakashi not to abadon his friends and give him his sharingan to perfect the chidori. Obitos dead.
i dont think any killing was involved in Kakashi unlocking MS but i agree he must have read those Uchiha tablets and learned about the sharingan eyes. Kakashi was a heavy reader. knowing the all ninja rules by heart as a kid and always reading those Make Out Tactics.
