Sasuke killed Orochimaru during their battle. Orochimaru was a part of team Hiruzen and was a part of the Legendary Sannin. Orochimaru possessed a complex personality.
Orochimaru was a part of team Hiruzen and along with Jiraiya and Tsunade. Hiruzen thought of Orochimaru as a prodigy that was only seen once in a generation due to his talents, insights and determination.
During the Second Great Shinobi while they were battling Hanzo, who gave them the title of Three Legendary Sannin. This was as a reward for surviving their battle with him while the rest of their group was easily killed in the process.
However, that wasn’t enough for Orochimaru and he started learning about various forbidden cursed techniques. He even kidnapped a few shinobis and used them as guinea pigs to help him develop techniques to attain immortality.
Sasuke is the second born child of Fugaku Uchiha. He mostly grew up in his elder brother Itachi's shadows. This never stopped him from adoring his older brother and he never gave up any opportunity that he had to spend time with him. Itachi helped Sasuke become a better Shinobi by training him.
One day after one day after long as training when Sasuke returned home and he found his clan people murdered on the street. He then runs home to inform his family of the sight he saw, only to find his brother Itachi standing over the dead bodies of his parents. He tries to help and comfort his elder brother who in turn puts him in a genjutsu in which he repeated sees Itachi murdering his parent. He tries to run away but Itachi catches up to him and explains to him that he was not worth killing.
Since that day Sasuke started training harder so that he can fight his brother. He did want to become powerful, that is why he left the village and approached Orochimaru so that he can help him gain more strength.
The relationship that Orochimaru and Sasuke was not of a master and student.
Orochimaru always saw Sasuke as a vessel because he also had the Sharingan. Over time he observed Sasuke and wanted to see how he would turn out to be. It was more like Orochimaru had seen some of himself in Sasuke and was curious to know what decisions he would take. He just wanted Sasuke to become his dream vessel or an ideal host body. To do so, he even let Sasuke boss him around at points. He went specifically for Sasuke because he had firsthand seen the power of the Sharingan because of Itachi, so he thought that if he go for his younger brother he could obtain the Sharingan. Also, the fact that he could take over Sasuke’s body much more easily than Itachi.
Orochimaru was telling Kabuto about how he had set up Sasuke as his next host body. He said that it was because his now current host body was rejecting him. As soon as Kabuto left Orochimaru was attacked by someone, and it was none other than his student Sasuke. Sasuke tells Orochimaru that he is aware of his plans and expresses his disgust toward it
Sasuke strikes Orochimaru but he escapes and reveals his true form which is a snake with his head on it. He tries to steal Sasuke but Sasuke uses his curse mark to escape.
While fighting with Sasuke, Orochimaru starts remembering some moments from his past like how he found a white snake next to his parents and asked Hiruzen about it who said that it stood for good fortune. He takes in Anko as his apprentice, learns about forbidden techniques, joins the Akatsuki, etc.
Back to the fight, after battling for a long while Sasuke manages to put his sword through Orochimaru's snake form. Sasuke thought that the battle was finally over but he was wrong, some fumes started coming out of Orochimaru's body which turned out to be toxic and paralyzed Sasuke.
It allowed Orochimaru the time he needed start to his immortality Jitsu on a weakened Sasuke's body. In the meanwhile, Kabuto returned and was shocked to see a destroyed door. When he entered the room he saw Sasuke standing there, he was confused if it was the actual Sasuke or Orochimaru had successfully taken over the body. To clear his doubts Sasuke turns and activates his Sharingan and shows Kabuto what went down.
The flashback showed that when Orochimaru transported himself inside Sasuke's mind, Sasuke managed to reverse his technique using his Sharingan. He told Orochimaru that his Sharingan was able to see through his technique and proceeded to absorb Orochimaru.
After the fight's flashback is over, Kabuto can't wrap his head around Orochimaru being gone and Sasuke leaves the hideout to continue fulfilling his goal.
A. Sasuke is more like an anti-hero who is the second most important character in Naruto.
A. Yes, Sasuke turned to a new leaf and that is due to Naruto not giving up on him even on his darkest days.
A. Now that Sasuke possesses the Rinnegan he can turn into an immortal if he wishes to do so.
A. During the chunin exams, Orochimaru gave Sasuke a cursed seal. He had done this to obtain Sasuke as his next host body.
A. After a lot of body modifications, it has been seen that Orochimaru’s true form is that of a giant white snake.