Darker Side of Eru

Eru is all, therefore Eru is evil, But Eru is also good. How does he manifest himself as both? He doesn't, The Darker Side of Eru corrupts even the strongest, Melkor being a prime example.

What is the Darker Side?
The Darker side is Eru, just a side of him some would view as evil, The Knights of Eru Are ones who obviously follow the Lighter side, but some can be possesed by the Darker Side, This is the reasoning for such a low number of knights, those who are tested must be pure of heart and mind.