In "Episode 359 : The Night of the Tragedy", it was revealed that Itachi sought the help of Tobi to massacre the Uchiha clan. So the following questions arise out of this situation:
- Did he know Tobi's real identity?
- Why did he take his help to eliminate the clan (irrespective of whether his identity was known or not)?
- Why did he help Tobi by not revealing that he was the village infiltrator as Itachi was always working for the Village?
- Why did Tobi agree to help Itachi eliminate his own clan?
Ideally, each question here should be a separate question, but in this scenario all these are closely inter-related and thus have been posted as a single question.