There must be contrast in the world of linear illusions whereby straight linear Time may have event sequences that appear one after the other (including feedback looping) so that the feeling of "beginning" and "end" may be seen by the initiate. His realm is Eternity or No Time...we participate in projected or broadcast Time/Space so that we may attain and keep coherency or unconditional Love. To see the opposite in this projected life maintains the beam of Godhood the cursor or eye of God.
Evil is the opposite of Love...it is necessary to maintain the illusion that we participate in. Evil is Chaos...but it may also be a means of learning the needs of the next echelon of being...for Chaos is but unknown order experienced at a higher speed or realm. Without contrast, there would be no experience...no memory of anything...it is all part of the mix of being in Time...without the contrast there would be only duration forgotten.
Not that one is, but that whereby Is, is.