The reason God is everything, where God contains both light & dark, is that our reality is not what we've been lead to believe.

In the beginning of reality from the ancient ideas, like the Greeks for example:
Everything was random chaos (Nyx), and then Heaven was formed (Uranus), and from there came the Source (Chronos) - which created time & order.

We've been lead to understand later theology - that there is a God of the Living (Zeus/Zan), and a God of the underworld (Hades).

Where we've been taught to believe in one of them, when a lot believe in both, and think the Devil has took over this world, and the God of the Living has weakness.

The reality is everything is inside Chronos (EL - God Most High), and we're not meant to be worshipping the Divine Beings (Elohim), as that is henotheism/polytheism.