Honestly we don't know enough about how the MS works. I mean, how does killing your best friend give you such an ability? Perhaps it's just seeing your friend's death that does it, but since you can't guarantee being around for such a thing to happen, the best way is to cause the death yourself. And perhaps Kakashi HAS been able to use MS all this time, but since it wipes him out to the point of hospitalization, he's only ever used it against Deidara.

I'd be happy to install the battery for you. Pop over tomorrow and I'll get right to it. ^_^