The first story, an ancient pagan story still around, is that evil co-exists and is co-eternal with the Good. Good and Evil exist side by side as light and darkness, they exist in a balance and, thus, are eternally pitted against each other.
The second story, the Christian one, is that evil exists because something broke. Evil is a catastrophe. Creation was originally and primordially good but a Fall occurred.
And the third story is that there's no such thing as evil. There's nothing evil about childhood cancer diagnoses, death, suffering, or tsunamis. The cosmos is run by deterministic, material laws which are neither good or evil, they just are. "Evil" is just a word we use to say we don't like something. We don't like death and suffering and horrors, but they aren't "evil." This includes the moral "evil" and horrors caused by humans, because humans are governed by the same deterministic, material laws.
As best I can tell, those are the three stories we can tell about the origins of evil.
