Orochimaru

.From the series Naruto and Naruto Shippuden.
Orochimaru was originally a student of Hiruzen Sarutobi, the third Hokage,  as part of a three-man team, consisting of himself, Jiraiya and Tsunade. Even at a young age, Orochimaru stood out as a prodigy, his talents, knowledge, and determination were considered by his sensei a sight seen once in a generation.
According to Tsunade, Orochimaru had always been twisted, even back when he was a child. His sadistic attitude was presumably due to the death of his parents, which led him to study kinjutsu and eventually becoming obsessed with learning all ninjutsu. Jiraiya theorized that Orochimaru went down this path in an attempt to forget his painful memories. Sarutobi in particular had very high hopes for Orochimaru, often berating the clumsy Jiraiya and telling him he should be more like his team-mate. After becoming the Third Hokage, Sarutobi hoped that one day Orochimaru would succeed him.

While Orochimaru's ambitions did include becoming the Fourth Hokage, his eyes were set upon goals quite alien to the ideals traditionally associated with the title and emphatically embraced by his teacher. Hiruzen knew that "a shinobi's true strength will manifest when protecting something important", and the Hokage's role is to love and protect the people of the village as if they were a family. For Orochimaru, however, Hokage was nothing more than a potential conquest on the way to what truly interested him: power. This mindset influenced the image had of people around him, with a young Ibiki Morino seeing him as "something inhuman in human form".