Tobi claims to be Madara Uchiha. He tells Sasuke he isn't an enemy. He reveals a shocking truth: he knows everything about Itachi. Sasuke lashes out, his Amaterasu activating automatically to kill Tobi. Tobi survives, proving his power is beyond comprehension. The Tragic Truth Tobi begins to unravel the lie Sasuke lived. Itachi wasn't a monster. He was a martyr. He killed the clan to prevent a civil war.
Before the Leaf ninja can reach Sasuke, a masked man named Tobi arrives. He claims Sasuke for himself. He takes the unconscious Uchiha to a safe location. When Sasuke wakes up, he is bandaged and trapped. Tobi sits in the shadows, his voice changing from playful to chillingly serious. The Mask Slips Naruto Shippuuden (Dub) Episode 139
He spared Sasuke because he loved him more than the village. The Weight of the Truth Tobi claims to be Madara Uchiha