The thing about allegory is that it derives its claim to truth through more than mere assertion.
If I told you the story of Adam and Eve and renamed the characters Gary and Edith, made the snake a talking mongoose, and had them eat the Orange of Truth rather than the Fruit of the Knowledge of Good and Evil, not much has changed. Old Bible myths get rehashed and remixed into ‘new’ stories all the freaking time.
In fact, when modern peoples state that the Bible is a work of allegory and not history, they’re actually elevating it. History can only give you lessons through context, allegory doesn’t depend on it, otherwise changing the context would change the story.
If you stripped omnipotence from God and just made him some powerful being that made humans and wanted to give them the whole world in a garden, the allegorical meaning of the story won’t have changed a single iota, and in fact, it’s probable that the original composers of the story didn’t really have all of their omnipotence theology hammered out when they composed it.
So what does Genesis 1 have to say about evil? It’s pretty simple really, God gave us everything, and we decided everything wasn’t enough, we wanted to be more like God. You can generalize that. No matter how much you give people, it will never be enough for them.
This is what the Catholics called “original sin.” Warding against this impulse in the self to rebel against those that give us the world, that’s how we avoid being evil. Catholics didn’t think it was possible until God gave us an example to live by and under.
Think about it. The modern world has given us a world in which we can appease our every desire, our every ambition. But still there are people that want more. If there ever stopped being people that wanted more, then there would not be any evil, because nobody would be motivated to rebel against the world, personified in scripture as God.
Even if you reduced God to a ‘mere’ personification of an abstract idea, a character in a story, it doesn’t change the meaning of the underlying message.
Such is the power of allegory.