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During Sasuke's fight with Orochimaru he asks him why he insists on taking drugs and transfering bodies to elongate his life. I have started to believe that Orochimaru is really not that bad of a person, but is only a child who cannot accept his parents death. Jiraiya has stated that it was after Orochimaru's parents death that Orochimaru began acting strange.
The problem was that he was not a child anymore- had not been for a long time at the time of his death or capture in the sword etc. He was a mature adult.
What Jiraiya said was probably was felt by the Third too- that's why he hesitated when he found out about Orochimaru's evil researches. But Orochimaru had derailed of the path by then, beyond the control of the good.
Oro wanted to live for forever- so that he can learn every jutsu around and made sure that it doesn't go to waste with his death- instead he wanted to be alive to live with it. ( I cannot site the manga as this is from scattered statements in the story.)
Orochimaru was quite an intellectual seeing how good a judge he was of human nature and its failings too. A great sanin and research scientist himself, he was able to manipulate a lot of people around him. Most of his followers were at least partially with him of their own free will. He was amused at Kakashi for trying to save Sasuke. He let Anko go alive even after he trained it and during chunin exams as well.
Orochimaru's decisions were never impulsive, but calculated. He knew what he was doing. He also knew his own danger when he took Sasuke in and said so when Sasuke went to him for final battle. He took the risk because Sharingan had caught his fancy and he wanted it so badly that he thought it was worth the risk. Seeing that he operated on Danzo and gave him his sharingan from Shisui, I wonder what he was going through at the time? How did Danzou manage him to operate on himself and yet not try to steal the other eye at least then and there?
With Orochimaru it was not about good or bad. He didn't think in these terms. His philosophy perhaps made him look everything in relativity. He would let you go to your death if it amused him at the moment. He might save you on whim as well.
No one knows when a psychological defect may turn to evil- but once it has taken that path, that turning point is just for academic records. An average normal person would get over it with time. A positive person would try to do some good to make up for it. A negative personality would go on negative path sooner or later- how it started means little and of not much importance- it could have been anything..
Just my two cents...