We need to understand what Orochimaru wanted to achieve in life and that was to master all Jutsus, since he lost to Sasuke (he's disciple) and by watching from inside how fail his way was watching Kabuto copy him completely understood that his Teacher (3rd hokage) might have being right all along and if it's proven that even Sasuke can go from good to pure evil and back to good like it goes why can't he himself go good again, and continue accomplishing his dream, master all jutsus. And something else to add here he doesn't even need to captivate bodies since he can just get into Jetsu's bodies wich was a big problem for him to go good again (he needed to kill other in order to be kept alive). We don't know what would have happened with Orochimaru if the 3rd had chosen him instead of Minato as the 4th Hokage. Kishi seems to have a valid pattern for all his villian's, ALL of them have a reason to go evil and all have the essence to change.
Orochimaru was cut off by his teacher from beign the next Hokage and trying to stop his research, Gara as a Jinchuriki was learn to hate all, Sasuke hated his bro until he learned the truth the he hated the village, Obito by not be able to accept that Rin died and that Kakashi didn't kept his promise decided to change by Madara's plan the world, Madara couldn't forget that he had lost his family couldn't really stay and play it mister nice guy in the village.
All of them had a reason to turn evil and all where good naive kids before with the essence to find goodness again.