I can't find any reason why obito want revenge of his clan on the uchiha massacre.. Any ideas?? But at this moment writing this.. I thought about obito was telling itachi that he is madara.. So maybe obito said those words about the revenge because the uchiha clan has betrayed madara..
I don't remember exactly that well anymore, but Obito I know he impersonated Madara. Madara has left the leaf village. In episode 345 or episode 346 of Naruto Shippuuden it was already shown that Obito was "supposedly Madara". Madara then spoke through Obito and it was later said that Madara Uchiha was manipulating Obito. Maybe that's what this is about :)
These events are best seen in the flash back arc, Kakashi: Shadow of the Anbu Black Ops. Obito takes part in the Uchiha massacre on the request of Itachi Uchiha. Itachi asks him both as a way to mitigate against whatever this strange intruder is planning (by making a deal with him), and because he knows he needs help to complete this mission. It is implied the reason Obito cooperates is to collect Sharingan. It is shown that Obito has an extensive collection of Sharingan, and the most reasonable explanation for is that he collected them during the massacre. You can see this very large collection at the end of episode 216 High-Level Shinobi:
Since he's not actually Madara, he doesn't actually have any kind of grudge against the Uchiha, so this is the only explanation that makes sense anyway. It isn't really feasible that he's acting out Madara's grudge since it was pretty much shown
He had no intention of actually resurrecting Madara, and was just going to carry out the plan himself.
Unless he for some reason felt the need to stay in the character of Madara, and to protect his guise wanted to take it as far as killing all the Uchiha. I don't find that extremely likely, as there isn't really an incentive to do so, but I suppose it's a possibility. I'll note that as far as I recall, isn't actually clear whether the real Madara wanted to exterminate the Uchiha though.