Alright here is my theory on Tobi,Obito,Madara and Itachi.
1.We all know what happened to Obito, he died on the rockslide.
2.Madara Uchiha, wondering the woods, found the dead body of Obito, he probably healed it, and kept it safe for his next body transfer.
3.The body he was using atm probably had enough strength to do a sacrificial-kyubi-summoning-jutsu to bring chaos to Konoha, since he probably hates them so much.
4.He body transfered to Obito since his old body was about to die.
5.He wasn't satisfied with just one sharingan eye, so years later he decided to try to take someone with 2 sharingan eyes.
6.Itachi had just joined ANBU, in one of his missions he encounters Madara and fight each other, either a draw or Madara is not strong enough to beat him since he only had had that body for like 6 years or so.(Itachi joined ANBU about the same time Sasuke entered school right? so thats like 6 years after the Kyubi attack on konoha since Sasuke and Naruto are the same age) Itachi learns dark secrets from Madara during his fight like how to obtain the Mangekyo Sharingan and probably something evil about the Uchihas that makes him hate his own clan, also learns that Madara wants to get a stronger Uchiha body, and there is no way to stop him unless there are no Uchiha's to body transfer to.
7.Itachi tests if he was telling the truth about the Mangekyo Sharingan and eventually he obtains it, making him realize everything Madara said was true.
8.Itachi, in order for Madara to not become stronger, decides to wipe out his entire clan except for his brother that he loves too much, and probably though that if there was one survivor, all of Konohas eyes will be focus on him and Madara will not be able to get close.
9.Itachi joins akatsuki in order to become even stronger and to try to find Madara, he also didn't tell konoha his reason for wiping out his clan, cause one, murder is murder, 2, maybe if he mentions Madara Uchiha is back, it will cause too much commotion on the village.
10. He learns that hatred can make Uchihas much stronger so thats why he wants his brother to hate him so much, so he can become stronger and hopefully stronger than him so they can both fight Madara.