Honestly I dont think he sold out, i just think he gave up. Somewhere along the line he lost faith in his story or just lost faith in himself as a mangaka and threw in the towel. He let the studio and the company drive his story and we ended up with the atrocity we got.
I think it's the other way around. He gave up...then he sold out. After the Pain arc, there seemed to be some aimless direction, along with heavy focus on Sasuke. There was no well-defined enemy looming (like Akatsuki had been), and all the forward momentum evaporated. I think this is where he lost his drive and just wanted the story to end. And why wouldn't he? It's clear that the story was built to end with the Pain arc. Anyway, I think SP/his editors came to Kishi with a weak plot, the offer of others to write/illustrate/carry the franchise, and buckets of money. And he took it. I don't think it was a malicious, just that he sold out after he'd already given up on the story. So that way, an NH ending looks like he's really rescuing a story that was going nowhere fast, and reinvigorates a plot that had already gone very stale in the author's mind.
If Kishi sold out, why pissoff his fandom instead of him praising his pairings and characters? That's what a sold out author would have done. What Kishi has been doing the entire year is the opposite. Its more like he's shooting his cows instead of milking them properly.
I still think Kishimoto purposefully destroyed his story. The reason behind it is I have four reasons.
1. He got tired of writing Naruto period seeing how he pretty much lets anyone else write the damn thing.
2. He really was lied to by SP and noticed how his fanbase acted towards it. So he is basically trying to demolish it as pay back so SP can't make money off of it. (Similar thing happened with Akumatsu over Negima)
3.He got mad at us, the fanbase, for caring too much about pairings than the story and decided to just give up on it.
4. He is so depressed and miserable with his life that he is on a path of self-destruction.
Yes...his negative quotes after the ending really were hard to take. And no fanbase was spared. It makes no sense. If he was happy with the ending (and with all his money from it) then why didn't he just let it go? I think it's a bit of #1 and #4: I think he got tired of writing Naruto a long time ago. And somewhere in his mind, he's ticked at the fanbase for being hung up on romance instead of action.
One thing I think is certain, for an author so invested in the bonds between Sasuke and Naruto, a huge romantic movie finale about Hinata and the whole manga ending hinging on the gimmick of which pairing wins was probably never in his plans. So even though he's reaping the benefits of that, I think it comes out in his quotes that he essentially feels like his story was twisted into what the fans wanted instead of what he wanted. That's the only explanation that I can think of for the absolute sabotage of his main characters in the end, and his sour comments that have followed.