Word of God: The creators told us what the big plan was for the show. John was really Sam's dad. He was originally a demon who fell for Linda (the "sickness" he came down with was love) and made a deal with Satan to be turned into a human. However, the Prince of Darkness couldn't transform him completely, which is why he couldn't die. One of the conditions of the deal was that he could never allow Sam to learn the truth, hence why he burned the pages referencing who his father was. As the son of a demon and a human, Sam was a candidate for the Antichrist, but he also has the potential to destroy the Devil, which is why Satan keeps him close: He's trying to influence Sam to become evil, thus guaranteeing that he becomes the Antichrist. Knowing this, Steve's comment in the series finale that breaking his hand was for the greater good: God was planning to have Sam remain close to the Devil, ensuring that he could gain his trust and stay close enough to destroy him.