One of the most endearing characters I've every played started out as neutral good- but do to a series of unfortunate circumstances slowly progressed to evil. You see, he started out as a free monstrous creature, and then became enslaved. For several years, he was mistreated and abused by humans. He managed to break free, but he became stranded in a strange new land with no knowledge of his surroundings or how to return to his previous life.
At that point he was justifiably wary of humans... this fear was repeatedly driven home after he attempted to gain assistance at a farm. Out of desperation, he kidnapped a human wandering the roadside (a homeless person), to force/coerce the human into teaching him common. The homeless man was of no use... but for the first time, my character felt a sense of control... for the first time he realized he had a way to exact revenge on the evil humans for stealing his life from him.
He killed the human, and committed his life to destroying humanity and freeing other beings from their tyranny.

After destroying several farms, a band of adventurers attempted to kill him... and nearly succeeded. With their weapons, treasure, and equipment, he began to gather and arm minions who would join him in his quest... mostly goblins.

At one point he realized that not all humans are the same.. but his preconceptions about them as a species and his own selfish desires for revenge prompted him to wipe out the vermin.

Too bad the campaign ended too early... I can think of a good ending for it.
Being killed by a paladin of freedom.