WTF!? HAS EVERYONE FORGOTTEN THE ENTIRE MADARA/HASHIRAMA FLASHBACK!?!?
He's not a goody-two-shoes.. He's still the bad guy.
He's going to do what Madara failed to do.
And that is killing his best friend and reclaim the title of Hokage, which he believes to be the Uchiha birth-right.
The one thing he's trying to restore ever since Chapter 003.
And don't forget; Naruto wants to become Hokage too, so they will have to fight for it to the death.
So, don't get your panties all twisted just because he said he wanted to become Hokage.
It was the next logical step really, very rushed but it does make sense, instead of letting his brother's sacrifice go to waste, he's going to change the village, him turning good without any other reason would make less sense.