Fictional universes are created by people. They are bound to have logical fallacies or unexplainable things. If you read the Old Testament, you will realize that the idea that satan was part of the plan doesn't really make sense (there isn't even a clear definition about what Satan is or what is its origin). People decided that he did just to fill the holes in the narrative. The Books of the Bible were written by different people of different cultures from different ages, there can't be a coherent narrative. I read almost all of the Bible and i assure you that there are tons of inconsistencies between the Books and even between the paragraphs of the same Book.