I believe sasuke turning back to the good side started a long time ago. I felt this was coming as it was a shock to everyone else. I believe it began when itachi touched his forehead and whispered something to him(maybe not in the manga but I saw this in the anime). I began reading naruto manga about 14 months ago after I watched all the anime to the current one.
Okay back to what I am getting at, I think this sasuke changing to the good side has been a long time coming. First when killed itachi. Soon after that he had a fight with danzo and encountered naruto and sakura after killing danzo. Naruto had a meeting with him and I believe this changed it further when him and naruto clashed and "met with fists". After that encounter with naruto he wanted itachi's eyes. Before that I think he did not want them because they were his brothers. After the encounter with naruto that meeting was burned into him. He started changing even more, sasuke started doing whatever tobi aka madara aka obito to see what was happening and to gain information. He wanted to gain information about his brother, as to the real truth. He could have easily killed bee i believe but I think he knew that he only hard a part of bee but didn't care he knew they wouldn't know the differnce until it was to late. As he was already planning on changing sides.
Soon after that he met itachi again and spoke to him he was already fighting inside as to what he really wanted to do. With itachi's eyes as it stood he could have went to the leaf village and massacred it. I believe if he still truly wanted to destroy the leaf he would have done so the moment he got his eternal sharringan. At that point he's already considering and doubting his resolve at his vengance. Then low and behold he goes and talks to the old hokage and well now he's where he is now. Hes a good character. I believe this has been coming a long time and it was no shock to me when he finally decided to come help naruto and act for the sake of good.
Okay back to what I am getting at, I think this sasuke changing to the good side has been a long time coming. First when killed itachi. Soon after that he had a fight with danzo and encountered naruto and sakura after killing danzo. Naruto had a meeting with him and I believe this changed it further when him and naruto clashed and "met with fists". After that encounter with naruto he wanted itachi's eyes. Before that I think he did not want them because they were his brothers. After the encounter with naruto that meeting was burned into him. He started changing even more, sasuke started doing whatever tobi aka madara aka obito to see what was happening and to gain information. He wanted to gain information about his brother, as to the real truth. He could have easily killed bee i believe but I think he knew that he only hard a part of bee but didn't care he knew they wouldn't know the differnce until it was to late. As he was already planning on changing sides.
Soon after that he met itachi again and spoke to him he was already fighting inside as to what he really wanted to do. With itachi's eyes as it stood he could have went to the leaf village and massacred it. I believe if he still truly wanted to destroy the leaf he would have done so the moment he got his eternal sharringan. At that point he's already considering and doubting his resolve at his vengance. Then low and behold he goes and talks to the old hokage and well now he's where he is now. Hes a good character. I believe this has been coming a long time and it was no shock to me when he finally decided to come help naruto and act for the sake of good.