you guys are substituting any random character for the real killer and coming up with theories yet you guys don't/can't explain why that killer would kill the uchiha clan.
sure i could say the character X killed off the uchihas, but that doesn't mean shit if i don't say why he/she did it. it's completely useless to tell us about a theory if you're not gonna explain what the main motive is for the killer.
from the way i see it, no one who has been introduced is strong enough AND evil enough to slaughter a whole clan except orochimaru and itachi. also, it can't be kabuto since i believe is around 17 (just a guess from appearance and when kakashi said "don't mess with an adult") and there is no way he'd be strong enough at the time of uchiha massacre (i don't think he is even strong enough now). and i doubt that the real killer is gonna be someone who hasn't been introduced yet. that would be completely stupid to just throw in the real killer after so much story line has been built.