Why did Orochimaru quit Akatsuki? It couldn't only be Itachi. Or was it?
Formerly, he did illegal operations inside the leaf village. Conspiring against them. All because they chose a different fourth Hokage. Not him. That really pissed him off them. He kicked a dog on the way home.
Then he was busted by his sensei. Uh oh. Naked human bodies chained to the walls. All of which was dead. How was that creepy? It wasn't like he was a necrophiliac.
Orochimaru managed to escape with his life. Luckily for him, the old man still had some hope for him.
Then there was Akatsuki. He was paired with Sasori. The man inside the puppet. He never saw his face and Sasori was just as psychotic as him. Overall, he didn't mind him but Itachi caught his interest with that Sharingan of his.
Sasori and Orochimaru carried on other operations outside of Akatsuki. Tampering and learning secrets about the human body. All in all, it was a pretty normal life. Itachi and his Sharingan still interested him.
Akatsuki started acting strange around him. Sure people normally acted differently whenever he was present but this was just strange. Too weird.
He decided to use Itachi as a host but Itachi started to become even more harsh with him. Everyone was treating him differently. On top of that, he and Sasori still didn't know what their Bijuu were. Everything was just one giant problem.
Finally, Orochimaru couldn't take it anymore... the horrific suroundings, the snarling beasts he'd face every minute, The biological nightmares and the unending droning... He finally decided to quit his job at McDonalds. (Every villain needs funding)
Of course, Akatsuki was a different ordeal. He couldn't take on Itachi's strength. And so he left that too. That's it. Orochimaru left the single most badass organization because of Itachi.