Well in that case, evil must certainly have had a great amount of power for a very long time considering there is much evidence indicating humans have killed and fought each other for basically our entire existence.
A lot of the "evil" also appears to be quite subjective. The supposedly "good"...
If we assume that God and Satan (or what you'd like to call him) actually do exist... what are the disadvantages of the latter? What is there to fear, really?