What Zoroastrianism says about good and evil
This is the tradition that made good and evil the central question, and a good deal of later thinking about the devil starts somewhere near here. Two points are easy to miss. The destructive spirit is not a second god but a created spirit who chose badly, and his time runs out. Ten concepts follow, from the first choice to the last.
The Two Spirits
At the beginning two spirits declared what they were, one choosing truth and the other falsehood. Everything else in the system follows from that pair of choices.
Gathas, Yasna 30.3
Angra Mainyu - The Destructive Spirit
The destructive spirit, later called Ahriman, is named as the source of disease, death and suffering. He is a created being who chose evil, not an equal and opposite god.
Bundahishn
Druj - The Lie
Evil has a single name here, and the name is the Lie. It covers falsehood, disorder and destruction together, which makes lying a cosmic matter rather than a small one.
Gathas
The Creator of All Good
Ahura Mazda is said to make only good things. Nothing harmful is traced back to him, which is the move that keeps the creator clear of blame.
Avesta
Origins of Suffering
Suffering is explained as an invasion of a good creation rather than as part of its design. Disease and death are damage, not features.
Bundahishn
Human Choice
The cosmic argument is repeated inside every person, and each one decides for themselves. The instruction is to hear clearly first and then choose.
Gathas, Yasna 30.2
Humans as Allies of Good
People are enlisted rather than watched. Right thoughts, words and deeds are held to strengthen one side of the struggle.
Traditional
Ethical Dualism
The two sides are not evenly matched. Good is expected to win because of what it is, which makes this an ethical dualism rather than a balanced one.
Commentary
The Limited Time of Evil
Evil is given a beginning and an end. The struggle is a period, not a permanent condition of the universe.
Bundahishn
Victory of Good
At the end evil is destroyed rather than contained, and creation is renewed. The outcome is treated as settled, which changes how the present is meant to feel.
Eschatology
It is worth noticing how unlike a balance this is. Zoroastrianism names two sides and then refuses to hold them level, because one is a corruption of the other and is going to end. That is how the same tradition can be so severe about the Lie and so cheerful about the future.