Ok, I don't claim to be that knowledgeable in christianity, but here's a theory:

According to the Bible "not to be taken literally" God made all and saw that it was good. Then adam and eve show up in the garden of eden. For awhile all was good in paradise, then eve grabbed the apple. I think this symbolically shows that we need something to contrast the good, or it means nothing. So the world changed from paradise to what it is now.

The question is: where did the evil come from? Did god create it or did it evolve by itself as a natural balance like yin and yang? If somebody more knowledgeable in christianity knows I'd like to know the answer.

My guess would be to say that it evolved that way. Lucifer was once the Morning Star, the right hand of God until he changed and became Satan. So what happened there? Did God cause him to change to become a balanced force, or did Lucifer naturally change his mindset.

(This post is getting really wordy but here's a last thought. According to the Bible Lucifer was God's right hand and he aspired to be more so God cast him out. Could it be that since he was God's most trusted, God entrusted him to be his "opposing" force?)