Short answers, pulled from the story.
Good and evil is a common dichotomy in philosophy, religion, and psychology. The principal study of good and evil, or morality, is ethics, which has three major branches: normative ethics concerning how we ought to behave, applied ethics concerning particular moral issues, and metaethics concerning the nature of morality itself.
Zoroaster was a Persian philosopher who simplified the pantheon of early Iranian gods into two opposing forces in conflict. He named them Ahura Mazda, Illuminating Wisdom, and Angra Mainyu, Destructive Spirit, whose struggle was to be resolved on a Day of Judgement.
Thomas Aquinas, the Dominican theologian, defined evil in the Summa Theologica as the absence or privation of good. The Catholic Church draws its understanding of evil from this definition.
Drawing on the Stanford prison experiment, Philip Zimbardo suggested in 2007 that people may act in evil ways as a result of a collective identity. He published this hypothesis in the book The Lucifer Effect: Understanding How Good People Turn Evil.
Friedrich Nietzsche rejected Judeo-Christian morality in Beyond Good and Evil and On the Genealogy of Morals. He argued that the natural, non-good had been transformed into the religious concept of evil by the slave mentality of the masses who resented their masters, and he claimed many who consider themselves moral are acting out of cowardice.
The Baha'i Faith asserts that evil is non-existent and is merely a concept for the lacking of good, just as cold is the state of no heat and darkness the state of no light. The Ahmadiyya understanding of Islam similarly holds that evil has no positive existence in itself and is merely the lack of good.