He found out that Itachi wasn't the monster he thought he was after he killed him, and how he was manipulated to kill his clan. So, he decided to destroy Konoha to avenge him, and what better way than with the Akatsuki?

You happened to be in the Akatsuki for a while, and they trusted you. You're powerful, have experience, and of course, you've proven loyalty.

So, they put you in charge of showing him the ropes.

"Who exactly are you?"