When it comes to the topic of why's, when's, how's and what's about the abandonment of the village by Orochimaru, there is a ton of important details about the events that are unexplained, and thus, unclear, in the story.
You see, 'evil' is a relative term. It is not in human nature to be born evil.
'Evil', here, in this series, is Orochimaru's countless atrocities - ranging from the creation and application Curse-Marks and 'unethical' human experiments to kidnapping of clan offspring and line-theft.
Now, it is clear in the Manga/Anime that Orochimaru wants to have any and all knowledge within reach or 'learn everything the world has to offer'. Now, this can be the main root of the acts of stealing Sharingan and experimentation on recreation of Mokuton.
Another such instance is pertaining the demise of Orochimaru's parents when he was about ten, or so, years old. The incident that took those lives were not specified in the story. Some of the widely-accepted assumptions are of his parents conducting illegal actions in-village (and everything was kept hush-hush), an infiltration mission going wrong, silent execution as a result of apparent insubordination or were prisoners of war. Despite the details being blurred around the edges, Jiraiya stated and I quote, "Orochimaru changed after his parent's deaths. He deeply involved himself to find a jutsu to bring them back." Some say that this tragedy was the turning point of the 'evil'-ness that originated in him? To want ones parents back is not a sign of 'evil'-ness, as they call it. Rather, it points into the direction of badly-handled trauma and lack of grieving period after the said deaths. After all, Senju Tobirama, himself, was one of many such cases - why does Orochimaru be called abnormal for that?
It can also be argued that for all Orochimaru was Sarutobi's 'favorite' and 'prized' student, – if you can even call it that – Orochimaru was always kept at a distance with a wary eye on him. Not too close, for Sarutobi's own 'comfort' and not too far away where his student could be monitored easily. It also says, from the Third Hokage's behavior that Hiruzen was unnerved by how his student was 'unnatural' and 'ghostly' and too intelligent to be true. Not to mention, the snake summons, the misunderstood creatures and Orochimaru's Clan Summons 'were far too much of an oddity in the village'.
The villagers, those buttheads, thought they had the right to judge people, namely Orochimaru with the other two being Hatake Kakashi and Uzumaki Naruto (but that is a story for another time), but if it was not for them, the Missing-Nin would not be like as he is and be all alone just because of prejudice and suspicion and whatnot.
The isolation, in case of Orochimaru, is what brings Shimura Danzō into the picture. Despite it not being clear in the story when and how the Snake Sannin started to experiment and discover on Danzō's orders while thinking they were from the Hokage, the details of the Warhawk interrupting ohaka mairi– at the graveyard where the Sannin's parents lay – to inquire about an 'emergency' was a total lie. Paying respects was a private time spent grieving for the loved ones but Danzō clearly had little to no boundaries. And thus, at the moment of vulnerability, it is seen that Danzō wants Orochimaru to agree to whatever is asked of him that he will, most certainly, not agree to anywhere else or in any other situation.
The arguments for Orochimaru's apparent corruption hailing from the experiments carried out on children are quite popular. But, again, it cannot be guaranteed that he conducted those of his own volition. It can be an order from higher authorities, namely the Fire Shadow and the village's Elder Council. The parameters of the mission aside, the health condition of the said children. Whether they were healthy and just kidnapped from their ideal environment, or they were physically crippled, i.e. missing appendages and such. But it does not excuse the fact that he was with unwilling experiment subjects. Alas, the uncertainties will remain no matter what.
Also, if we dig deeper into technicalities, the orders/mission of experimentation of the level of Mokuton (which is clearly line-theft), are obviously S-rank and thus to be destroyed after reading the mission parameters. If that were true and Orochimaru destroyed the only evidence to his orders, whether it had the Hokage's seal on it or not, it is no wonder that he was easily made the scapegoat where his curiosity made him pay a great price, which in this case is, of leaving the village and his home. While it is not certain that Sarutobi knew about the experiments or was informed of them by Danzō only, it can be either a betrayal or a misunderstanding all the same.
From a different perspective, it can be said that when Orochimaru was approached by Danzō, he discovered the secrets of Danzō's Ne or ROOT. Following this line of thought, it is a possibility that Danzō and his ROOT agents were somehow able to overpower Orochimaru in both politics and combat. Thus, leaving Orochimaru to confront Danzō or somehow be caught in the middle of gathering evidence seeing as Shimura, first and foremost, had the ear of Sarutobi and therefore, he and the Clan Council were less likely to believe the Snake Summoner without said evidence, if at all.
Following the above paragraph, it can be said that if Orochimaru was, possibly, caught what with Tsunade and Jiraiya being out of the village and leaving him no in-village support at all, it is probable that Seals – being one of old Warhawk's specialties – were used on the Sannin to keep him 'obedient'. The seals could be anything – be it a slave seal (like the one on Uchiha Obito and Nohara Rin's hearts), one to keep Orochimaru's silence on 'undesirable' information, or a seal to lock his emotions or the uses of mind control techniques and genjutsu such as the Kotoamatsukami. This predicament would also ascertain the reason of using such a vile technique as the Living Corpse Teachnique, where one can possess the body of another and use it for themselves, in an attempt to nullify the Seals etched onto him. It can also be possible that the Seals were etched onto his soul as it does in case of a Summoning Seal. And after the war, when Danzō was dead, Orochimaru had role reversal of 360°.
My reasoning of this analyzation revolves around the fact that Orochimaru once had parents, a team of Genin, Anko – a 'family'. That he is also a human and that he, too, has emotions what with his parent's deaths, Nawaki's death, his friends and family's abandonment – this all breaks a person, so how it is that Orochimaru is any different? Seems a bit far-fetched, doesn't it?
While it is entirely possible that Orochimaru had been truly 'evil', because there is no possible way to know what happened in his hideouts and what his mannerisms and actions were or if it were done of his own free will, I like to think of the plausible events that shaped Orochimaru to be the way he is. As for the bad choices that the Snake Sannin made, well... everyone does at some point.
