Uchiha's wanted to take over Konoha because of some stupid bullshit that happened 80 years ago. They used Madara to sick the ninetales on Konoha but got their shit ruined by the fourth. Madara's power was useless after this somehow so they needed another mangosharingan to bring it back. Itachi was such hot shit that they decided to make him the new top dog on campus. They were gonna make Sasuke the eye sacrifice for Itachi's perfect mango but Itachi was like "aw hell naw" and told the Third about it. Sasuke would probably have sided with Uchiha and turned on Konoha if he found out, so Itachi kept him ignorant so that the Third would allow him to live peacefully in the village. So Itachi and Madara (now disowned for being useless) killed his clan and left some parting words to Sasuke.