Evil is simply the corruption of good. God gave the beings he created the gift of free will so that they would not be mere automatons. Inherent in free will is the ability to choose to do something against God's will. Satan was once good, but his ambition and desire to have power separate from God's caused him to be cast out (or, some would say, cast himself out) of Heaven. We have the same free will, and we have the same choices to use our power for good or for evil. I highly suggest you read "Mere Christianity." It explains things much better than I ever could.