As to all those saying Madara ordered Obito to participate in the massacre, you guys give him way too much credit.
I don't think Madara took the time or effort to tell Obito to kill his clan, there was too many other things he had to tell him. Let me try to explain myself as well as I can...
Let me be clear that Madara did not know about his clans intentions near the time of the massacre because he died well before it occurred. It was not a matter of Madara telling Obito to kill his clan and Obito happening to agree once the random chance occurred...... do you understand how illogical that is?
1. Madara had no idea the Ku-de-tah (or however you spell it) was ever being formulated but DID hold a grudge against his clan.
2. Itachi, BELIEVING Tobi was Madara, was convinced that Madara would jump on the opportunity to get revenge on his clan, given the opportunity.
3. Keeping up his image as Madara and not caring about the individual lives of his former clan, Obito willingly agreed to the massacre because he was solidifying his role of Madara IN ITACHI'S eyes.
I feel that this is the only way to look at this, Madara had no means to organize this when it occurred around 10 years after his death.