Angra Mainyu is the sharpest expression of the moral dualism that distinguishes Zoroastrianism from the mythologies around it. Where other traditions locate evil in a rebellious creature, a fallen angel, or the by-product of a good creation, the Gathas and later Avesta present the destructive spirit as a primordial principle in its own right, coeval in some formulations with Ahura Mazda and choosing, at the origin, the Lie (druj) against the Truth (asha). Every noxious thing, disease, winter, predation, death itself, is read as his counter-creation, deliberately introduced to spoil the good world. The system is not symmetrical, however: the tradition is emphatically not that good and evil are equal and eternal.
Angra Mainyu is destined to lose. At the Frashokereti, the final renovation of the world, he and his works are undone and the creation restored to its intended perfection. Later Zoroastrian theology, especially the Zurvanite current, wrestled with how two opposed spirits could arise, and it is this problem, the origin of a genuinely independent evil, that made the figure so influential on later Jewish, Christian, and Manichaean thought.