Now we all know that it was Obito who helped Itachi massacre the Uchihas. But what could be his reason?

1. So he could recruit Itachi?
I don't think this would make sense because the Uchihas are starting to rebel against Konoha. And with a little persuasion and a little acting the he's Madara, he could recruit all of them. And the whole Uchiha clan is definitely stronger than a single Itachi.

2. To steal their sharingan?
Doesn't make sense also. As I have said in number 1, it's probably easy to recruit those Uchihas at that time and sharingan eyes in an Uchiha body with an Uchiha mind that is rebellious against Konoha is much much better than eyes alone.

3. To prove that he is Madara and he wanted revenge on the Uchihas?
Nah. It wasn't revealed that Madara is "alive" until the Itachi and Sasuke fight.

4. To convince Itachi that he is Madara?
Why would he need to do that?

5. To force Sasuke's strength growth?
Doesn't make sense.

So what do you think was his reason for this action?