Board Thread:Theories and Speculation/@comment-28456867-20160525000548/@comment-26773508-20160527080624

BeyondRed wrote: Probably the same reason he unleashed the Nine-Tails on Konoha, took control of Kirigakure, participated in the Uchiha massacre, and went out of his way to ensure Sasuke was on his side. Not sure what that reason is, mind you; the series never told us why he did anything. The series hasn't said anything outright about why Obito did these things. Sometimes, his actions seem pointless. But I'd venture a few guesses:

1) Kyuubi attack on Konoha :

I think Obito wanted Kurama to be sealed inside a weaker jinchuriki. He knew Kushina's Uzumaki abilities. And with Minato at her side, extracting Kurama would be virtually impossible if he didn't strike at the right opportunity i.e. during childbirth. So he killed two birds with one stone with that Kyuubi attack: Kurama got sealed inside an infant and Minato died.

2) Participation in the Uchiha Massacre :

Obito obviously had a boner for spare sharingans. I suppose the Uchiha Massacre acted as a sharingan harvesting season for Obito.

3) Obito trying to corrupt Emo Sasuke into the Dark Side :

Obito's plan was to use Sasuke to take the 5 Kage as hostages, because apparently he couldn't do it himself. And if that failed, I guess he planned on using Sasuke to attack the Leaf, either before or during the War. That would weaken a strong power like Konoha to a certain extent, and thus make his Eye of the Moon plan go smoothly.

4) Controlling Kirigakure :

No idea. Maybe Obito used to get a huge boner watching comrades kill each other. Or maybe he secretly siphoned off money and resources from Kirigakure to Akatsuki.

EDIT :

Now I haven't read the Itachi novels, but maybe Obito spared kid Itachi's life so that he could harvest his sharingan later. Didn't the Itachi filler show Itachi unlocking his sharingan when Obito attacked the convoy? Maybe that was the reason.