The Byakugan's evil intend

[psukkar]

Personaly I doubt that the eyes ever change "sides" and they are just eyes, however powerful. The only thing I believe changes the evolution of the eyes is the person himself, if the person is evil he will simple learn dojutsu's that seems more powerful and evil.

We already know that MS can be awakened without killing your best friend/the person closest to you, because for Sasuke this is Naruto and he doesn't kill him, I too believe that this way of achieving it most likely is an "easier" way to do it or maybe just the way the Uchiaha's thought. Maybe the first one to do it awakened it after a power surge from watching he's friend die on the battlefield and later on people thought that this was the way since there was no one before him to get it and that was the way it happened.

About the EMS it's hard to say, so far no one have achieved it without the eyes of a sibling and there have bin no talk about it either. In most cases I doubt it have been through self sacrifice that people have achieved the EMS, but trough sher evil. I don't think Kishi plan to make it any other way, if he had planned to do it, Sasuke would have been the one to do it. Might even depend on how long the manga is going to last, if the war is the last of Naruto it will stay as it is, if we ever get to see another 200-300 episodes after the war, everything might happen, maybe our favorite generations kids will be the ones to reveal the truth(kinda joking, kinda not)

Sasuke's ms has nothing to do with naruto.

He awoke his ms after learning about itachi's life. There's no evidence that suggests you don't need to experience the pain of loss in some way at the moment.

The only way to know for sure is to learn how kahashi did it.

[Tobirama Senju]

To obtain an MS you need to experience the death of a person who is closest to you. You must feel the emotion of losing that someone, and killing them doesn't necessarily activate it.

It has been hinted that Kakashi's other team mate Rin has died. So maybe Kakashi suffered the loss of Rin, and thus awakening his MS?

[Gaara Kazekage]

Well, for all we know about Kakashi it might have happened anytime along the road. Most likely he got it during the time skip Original-Shipuuden. but he can also have awaken it a lot earlier and chose to keep it a secret until he really needed it.

It's like Psukkar say's, we can't know for sure how MS works until we know how Kakashi did it