Why is Orochimaru become evil?

Answer 1

As they grew older Orochimaru, Jiraiya, and Tsunade collectively became known as the "Legendary Three Ninja due to their exemplary ninja abilities. Orochimaru, however, craved more power than could be obtained from training with his master, and began abducting villagers of Konohagakure for experimentation. He simultaneously applied for the title of Hokage, the village's protector, hoping that the position would give him access to new abilities. The Third, though wishing to pass the name of Hokage to his favorite student, could not bring himself to knowingly satisfy Orochimaru's lust for power, and thus gave the position to Jiraiya's student Minato Namikaze, the Fourth Hokage.

so orochimaru became evil when he was not chosen to be hokage, all he wanted was power . so he left konoha because he NO longer having a reason to keep face in Konoha

Answer 2

Because he was doing that wasn't for the good of the village, he was, by the way, a Jonin before he left, but the biggest thing he did was that he planned to assassinate The Third Leader and make him forcefully Leader of the Leaf, but it was snuffed out, and he fled and made his own village, The village of sound.