Why evil? What is its existential and theological place?
Here's a metaphor.
But first, a bit about 'evil'.
Evil is the negation of God. Evil is 'not-god' in its entirety.
God calls us to fellowship with him, but we reject that. We reject the life of being with God and instead prefer the life of not-god, of living in rejection of God.
God respects the freedom we have to live in rejection of him; but that results in the experience of rejection of him. Without this experience we would not know that we were in rejection of God.
If you are married but live in rejection of your spouse (divorce) then you no longer experience your spouse. Its over.
So with God. However he provides us a way to turn to him. We only know we need this way because we know very well what the alternative is. The alternative is living in evil, living in 'not-god'.
Its as though we reject a bright light. We don't want to walk in bright light so we turn from him. Instantly we walk in shadow. With a bright light the shadow is a deep shadow. We can no longer see clearly. We stumble and fall, we can't tell shadow from obstacle, shadow from chasm. We make mistakes, we can't see so we don't know. We grope and find the wrong things, we step and trip. Sometimes to our death.
This tells us that being in the shadow is dangerous and we can't even make sound use of our eyes.
But turn to the light, and it all becomes clear.
Thus, we are called to turn to God. Christ makes this possible because by nature we love the dark. We love non-god.
In this world which is as a whole turned from God in every way, we, has new creatures in God will still suffer; maybe even more because we no longer call the world, the not-god, our friend.
But we have the hope that the bright light, life with God forever, awaits us. We know this because of his Spirit within us that tells us we are his.