Why is the devil so wicked?

You will have to take this answer on faith I am afraid. As we were all spirit children living with our farther in heaven, we also had our agency to think what we so desired. Satan or Lucifer as he was known in heaven, had it in mind to take the place of God. Because of this he was cast out of heaven when his plan to save all mankind by force was rejected. His one desire is to destroy the plan of God and have all mankind become as wicked as himself.