Why must evil succeed? Is it not possible for good to rule uninterrupted?
Ideal maybe, but unproductive.
Think of all the stories -in books or on the big screen- and tell me good's victory isn't so much sweeter after evil had gained the upper hand.
It's an integral part of storytelling! If there is not a mountain to climb and perils to struggle through, the journey isn't worth taking; for the one setting out, or those of us along for the ride. As a writer, if everything is handed to our characters on a silver platter, they would never learn or grow.
God works in the same way; you could say stories take their "cue" from Him. He allows evil to rear it's ugly head to remind us how truly horrid it is and make His victory even more incredible and life-altering. Growing becomes an inevitable product of this.
Look at Job in the Bible! He practically perspired blessings, and God allowed almost all of it to be taken away from him. It was ugly, sure. Life-changing, definitely! But the aftermath was much more rewarding. It was worth every second of the pain and struggle he endured.
It's difficult for me to see evil win in any form, be it fictitious or in the real world. But I take comfort in this; evil may have a hold in this world, but God holds the world in His hand. I say "Let Evil Reign" because its day is coming.
Good will triumph.