The secret behind the Sharigan (SPOILERS!!)
The kyubi mentioned how "sininster" sasuke's chakra is comparing it to Madara's ( and he was the baddest sharingan user since he is the 1st sharingan user). TO sum it up Madara made a pact with kybui so he can borrow his chakra to defeat his the Tengu God..which he barely deafeated him and sealed him up...
WHO is this tengu God? Well. he none other than uchiha Madara's Father!! what tengu did was diguise himself as a human, maried this girl form the hyuga clan had a boy (madara). as he was growing up he showed remarkable talent as a ninja, then his byugakan evolved into the sharingan. as his fater watched his son power ans skill grow. he made his true intention known! and that was to create a host to make him more powerful! thats why he had a son so he can take over his body. but madara realized he could never defeat him adn thus ended up making a pact with kyubi.
Now check this out.. Madara stated Tengu can be released if the 3 types of MS are activated at the same time. Itachi knows 2 that we know of and sasuke knows the 3rd one since he was going to use that one but oro stopped him. The suicide letter itachi forged about is best friend was his personal feelings regarding his clan. when he learned the secret of the uchia clan he felt their secret should be gloried not kept in secret/shame.
( OFF TOPIC!!Madara also had a best friend when in the end they ended up fighting.. just like naruto and sasuke.. those two statues where sasuske and naruto fought is madara and his best friend)