Why did Malthael turn evil?

Why did Malthael turn evil? He had beheld the latent power of the nephalem hero. He had seen that hero defeat the Prime Evil, an act that in all his years of war in the Eternal Conflict, he had never been able to match. Faced with such power, and beholding humanity as an abomination, Malthael resolved to become the embodiment of Death.

He had beheld the latent power of the nephalem hero. He had seen that hero defeat the Prime Evil, an act that in all his years of war in the Eternal Conflict, he had never been able to match. Faced with such power, and beholding humanity as an abomination, Malthael resolved to become the embodiment of Death.