The idea was to create a balance between Good and Evil, instead of Creating a conflict between the two. Good and Evil are inseparable.
When God gave Adam the Garden he told him not to eat of the Tree of Good and Evil, Life and Death. This proves that even in the beginning Good and Evil existed even in God's perfect world. One only exists where the other exists.
Good and Evil are like soul mates, always finding each other.
I hope you enjoy this work.