God new man would fall and already had the solution in advance - "the lamb that was slain before the foundations"
The reason that He created a contrast to Himself was so that He could be know in His fulness. What better way to show light that to show what the light looks like compared to the dark. Darkness is dispelled by light, and is the evidence of ultimate impotency. The scriptures do say that He foreknew that part of His creation was evil, and how better to show forth the light than with a contrast of darkness. I think this existence of evil is the only way that we are able to know and discern who God is and be drawn to that light and power.
God states through His Spirit to Paul that He suffers the vessels of wrath, where evil exists, for a time so that His power and wrath will be shown. God created evil so He would be discernable and knowable to His elect, vessels of mercy, and be called out.
This plan has been from the beginning so that God could create and have fellowship with a creation in His own image as FAMILY, like unto brothers, as Jesus will be our brother. After God accomplishes His will with man, evil is cast out along with those vessels.
Romans 9:22 What if God, willing to shew his wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction:
23 And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory,
Revelation 13:8 And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb slain from the foundation of the world.