he didn't
Sasuke killed Orochimaru himself ,well sort of.
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It's like 30 episodes long, starting when Naruto and Sasuke fought each other in the hospital to when Sasuke beats Naruto and leaves for Orochimaru.
Orochimaru isn't killed by Sasuke, he is just sealed inside Sasuke's body. When Shikamaru and Naruto kill Kakuzu and Hidan, Sasuke learns last technique from Orochimaru and realizes that Orochimaru is weaker than himself. During the battle between Sasuke and Itatchi, Itatchi draws Orochimaru out of Sasuke and kills him.
Naruto vs. Haku, Naruto vs. Orochimaru, Naruto vs. Kimimaru, Naruto vs. Sasuke, Naruto vs. Deidara/Sasori, Naruto vs. Orochimaru, Naruto vs. Pain.
Yes, Orochimaru does die. When Orochimaru tries to hold Sasuke in one place so Orochimaru can inhabit Sasuke's body and control the Sharingan he possesses, Sasuke uses the Sharingan against him and kills Orochimaru, all before his assistant comes back after he left to take care of some business.
Sasuke loses it in his fight with Itachi when they are fighting and orochimaru emerges from Sasuke in the form of a 8 headed snake and itachi uses the sword of totsuka to seal orochimaru for all eternity along with the curse mark.