The need for an Infinite Atonement performed on our behalf by an innocent godly being, Jesus Christ, comes from the eternal nature of laws and the necessity of penalties for broken laws. But there is another law that is required for God's plan to fully work in our favor. That is the law of agency. But it's not just agency, or the ability to freely choose, it is moral agency. Here are some links for more information on:
1) agency and atonement- 2 Nephi 2:27
2) moral agency- Doctrine & Covenants 101:78