There were two angels. One good and the other not so much.
Two angels that were very important to all of creation. Because of this, there was pressure on the older one. Soon this became too great, the expectations to live up to the legend of his name, (half the stuff you hear, they made up about him for publicity!), and to fill into his father's shoes because he had left the building. Because this angel was the oldest Archangel, he was the leader. Soon this angel couldn't take it, so he voluntarily fell. He ripped out his own grace and hid it in a pendant that would burn hot in the presence of God. He would wear it when he was reincarnated. So Michael fell. He became Dean Winchester, his own One True vessel.
The other angel didn't exactly fall to Earth, he was more like dragged out of the pit beneath the earth. This was the very angel who would not bow to humans, and this very angel was about to become a human. This angel grew lazy in his cage, so God decided on another path for him to learn his lesson with. God decided to make this angel the very thing it hated. So God ripped out the angel's grace, but because God had ripped it out and not the angel himself, some was left behind. And because of the way the new version of the angel's parent had died, that power was assumed to be Demon's blood within the body. And this was how Satan, or the Devil, or Lucifer, or whatever you want to call him, was given his final punishment. The worse punishment of all. He became Sam Winchester, who was destined to be his own One True vessel.
But the apocalypse was coming.
A.N. - I don't own Sam or Dean Winchester, Michael the angel, Lucifer the angel, or anything related to Supernatural, just my own characters or my own ideas that I use.