Monotheism: Monotheism is the belief in one god, who may have various agents under him. It believes that before anything, this one god, a force of light, came and brought into being all creation. One of his agents then rebelled against him and fell, thus creating Darkness. In monotheistic religions, the one god is ultimately good and the traitorous agent is ultimate evil.
Polytheism: Polytheism is the belief in multiple independent gods, along side a wide variety of magical races living alongside man. These mythologies usually state that all creation stemmed from darkness, represented by some primordial force (Jotun, Titans, Tiamat). Creation was often portrayed as wild and untamed. Then came the gods, who tamed the wild creation and thus brought light. The primordial force has become the antagonists of the gods, though the gods were their children. Polytheism seems to not display the same black and white concepts of good and evil as monotheism does. Good and evil seems to be highly subjective. We can see this in how the gods aren’t always good or wise and their antagonists aren’t always so malevolent. Darkness is the primordial force of creation, the chaotic untamed side of both nature and man which causes destruction but also new creation. Light however is the force that brings order to creation, taming the wild forces and stabilizing some form of order. The antagonistic relationship is necessary, for without the creative chaos, there will be stagnation. So the antagonists of the gods are needed so we may have balance.