Re: Sasuke , Madara and Obito .
Okay, that's way too much over an over simplification.
Sasuke did not become evil b/c Itachi. He first became evil for three reasons
1. Sasuke has always felt inferior to others. He deals with this be training and acting superior. When he learned he could never catch up with Naruto or Itachi, he just broke. That is the only way I can describe it. He wanted power and he wanted power now. He was tired of training and still having to be saved from Haku and later Gaara. He went to Orochimaru, for example.
2. Sasuke was motivated by filial piety. His clan was massacred. The murderer is still out there. He has to go fix that. His entire clan will continue to suffer if he doesn't avenge them. That is truly horrible pressure to put on Sasuke. That's how he justified killing Itachi and Danzo and planning to massacre the entire village. His wanting to avenge Itachi's death is a subset of this, since Sasuke sees his death as pointless. I'd beg to differ but that's how Sasuke sees it.
3. Itachi was a tool of Danzo. (edit: My other point included Itachi's death, but this point is independent of that, as it is about Itachi's life but it's perfectly valid to see this as a sub-point of 2.) Itachi was used by the Leaf and made to kill his own parents and become universally reviled for it. Yes, Itachi accepted this as his punishment for the massacre, but in Sasuke's mind that is not fair (Sasuke as a very idealistic and unrealistic view of justice.) Since it is nor fair, Sasuke wants correct it.
(I'm certain I've missed some other motivations but nothing else comes to mind right now)
Yes, Madara was consumed by hate of Izuna's death but there is more to than that
1. he felt the same way towards Hashirama as Sasuke felt to Naruto
2. he was promised the position of Hokage. Yes, he didn't deserve it. However, he was promised it and didn't get it. In his mind, this would be betrayal
3. Revenge for Izuna, as you mentioned
With Obito for me due to his constant flashbacks, his motivation is
2. see edits
I cannot think of anything else. Yes, he told Kakashi there were other reasons but they were went back to Rin's death and he constantly has Rin flashbacks, leaving me to conclude only Rin is motivation
I don't like this because it is too much of a simplification. No one is motivated by one thing, but Obito appears to be, which is why I think he is poorly written character. In characterization, motivation is everything.
edit: I suppose you could have Madara's teaching. However, he does seem perfectly willing to betray his master. I didn't include this originally because again he only flashbacks to Rin. I forgot
2nd edit: I suppose you could also add to his motivation list that because Rin died, Kakashi and his team betrayed him. however, it's still all goes back to Rin.
Kishimoto could have added to the complexity of the character by having him angry at being left for dead. That would have been a motivation completely independent of Rin's death. However, the flashback reveals he wasn't at all angry at that. He just wanted to go back the to Leaf when he woke up with Madara