Sam's visions -- they weren't drafts, they were memories. My memories. Other Sams and other Deans in other worlds. But guess what. Just like you, they didn't think they'd do it either. But they did. And you will too.
God, aka Chuck Shurley, conceives different worlds because He likes to write and create. He says to Sam that, "of all the Sams and Deans in all the multiverse, you're my favorite." He later reveals that He has manipulated the Winchesters' lives for His own amusement. Lilith reveals to Dean that Chuck has a "perv-y" obsession with the Winchesters and that every single one of Chuck's stories end the same way -- one brother killing the other.
When Sam shoots Chuck with the Equalizer, it creates a link between the two. Sam theorizes to Dean that since the gun does not fire bullets, he shot a part of himself into God, allowing Sam to see the different endings from Chuck's memories through nightmares (which Sam had initially dismissed as some form of PTSD). When Castiel tried to heal Sam's wound from the Equalizer, Sam caught a glimpse of one of these endings which he didn't understand.
During Chuck's monologue to a Radio Shed clerk on Earth 2, he shows the clerk parts of the Demon Blood Ending and the Samifer Ending on big screen TVs as part of his video presentation. Chuck then proceeds to destroy every alternate universe except the Winchesters' world to prepare for "The End", presumably including these worlds and any surviving versions of Sam and Dean and their allies.
These are the endings played out in Sam's visions.
Billie's plan is later revealed to have been one of Chuck's endings, His true ending, Chuck having used His power over time and space to manipulate events to lead to it. However, He once again fails. He then begins an ending where Chuck wipes every living thing on Earth from existence in order to make the Winchesters and Jack live with their failure to save anyone since they wouldn't give Chuck the ending that He wanted. Chuck went so far as to even reject their offer of killing each other if He would restore everyone else.
Ultimately, the Winchesters come up with an ending of their own where Chuck is left powerless and mortal and will have to live out the rest of his life, grow old, get sick, and die, and he will end up forgotten like all the humans he forgot about as there is no one left to mourn him. Chuck is left stunned by His defeat, admitting that He never foresaw an ending where He lost.
God. He is not exactly Shakespeare. He's more of a low-rent Dean Koontz. I had to listen to His whole "writing philosophy", and His very weird, very perv-y obsession with you. And it always ends the same -- one brother killing the other. I mean, this world? He could've ended it in so many ways, and He likes that one. You know, I guess that's why you had to see the werewolf bros die the way they did 'cause "foreshadowing".
Demon Blood Ending
An alternate ending in which Sam embraced his demon blood addiction and went full Dark Side, becoming the general of the demon army as Azazel intended. In this ending, Sam has killed Bobby and Jody, forcing Dean to pursue him with his own band of hunters (which included Benny Lafitte). After confronting Sam in the Bunker, Dean is killed when he has his neck telekinetically snapped by his brother.
An alternate ending where Lucifer was successful in possessing and maintaining control of Sam. Dean attempts to kill Lucifer using the Colt, shooting Sam in the back of the head. The Colt does not work, and Lucifer is easily able to heal the bullet wound in his head before burning Dean alive.
Mark of Cain Ending
An alternate ending where Dean has given into the Mark of Cain. After a one-sided fight where Sam attempts to talk Dean down, a Knight of Hell version of Dean picks Sam up by the neck and flashes his black demon eyes before stabbing his brother with the First Blade and killing him.
- Episode: 15.05 Proverbs 17:3
Butch and Sundance Ending
A possible future where Chuck had been successfully trapped. In this future, Castiel takes on the Mark to imprison Chuck, and is forcibly locked away in a Ma'lak Box after it begins corrupting him. Without Chuck around, monsters are able to build their ranks, causing the deaths of many of Sam and Dean's friends including Claire Novak, Donna Hanscum, Alex Jones, and Eileen Leahy. On November 3, 2021, Sam and Dean are turned into vampires, and have their last stand on December 22, 2022. Dean kills Jody, while Bobby decapitates the vampiric Sam, who was disabled by a dead man's blood bullet fired by Jody.
After Chuck leaves, Sam tells Dean that he believed Chuck's version of what would happen if they trapped Him and he still does.
- Episode: 15.09 The Trap
"Supernatural: The End" Ending
Named "Supernatural: The End" by Chuck, this is the ending he revealed to Becky Rosen in 15.04 Atomic Monsters, which she describes as "dark" and "hopeless". As Jack prepares to sacrifice himself to kill Chuck and Amara, Chuck reveals that this is His true ending, having used His control over space and time to manipulate the situation and Billie. Using Dean's rage and willingness to sacrifice Jack to get rid of Chuck and Amara, Chuck intends to force a confrontation between Sam and Dean that will end with one of them killing the other. Dean's betrayal causes Amara to agree to allow Chuck to absorb her. However, Sam is able to talk Dean down and Chuck once again fails.
Having failed once again, Chuck gives up on His plan and departs in a rage, stating that He no longer cares what happens.
- Episode: 15.17 Unity
The Rapture Ending
After failing to get the Winchesters to kill each other, Chuck systematically wipes all life from the face of the Earth aside from Sam, Dean, Jack and Michael. Though Michael survives this event, his vessel Adam Milligan dies as well. Not even birds are left and when Dean finds a dog he calls Miracle, Chuck sadistically wipes the dog from existence in front of Dean and gives him a taunting wave from across the street. The Winchesters attempt to get Chuck to reverse it by offering to give Him the ending that he wants with the Winchesters killing each other, but Chuck explains that His new ending is that they are left to suffer upon an empty Earth, having failed to save anyone due to their refusal to give in to Him.
Chuck resurrects Lucifer in order to get His Death book. Lucifer turns the Reaper Betty into the new Death, but kills her once the book is open and reveals his true plan. Lucifer is killed by Michael and the Winchesters tell the archangel that after reading the book with help from the Book of the Damned, they now have a spell that can destroy Chuck. However, Michael betrays the Winchesters and Jack to Chuck in an effort to become God's Favorite once again. Despite this, Chuck kills Michael for his previous betrayal of Him. Chuck then decides that he's bored with this ending and begins to beat the Winchesters to death personally. However, Chuck's plan backfires as the ritual Jack performed didn't just turn him into a bomb, but a power vacuum for divine energy. Each blow Chuck lands on Sam and Dean, Jack absorbs the power released until Chuck is rendered powerless against Jack, who siphons the rest of his power leaving him human. With Chuck powerless, the Winchesters and Jack decide to leave Chuck to live a mortal life, grow old and eventually die forgotten in an ending of their own choosing.
- Chuck's endings where one brother kills the other are reflected in the final battle between Michael and Lucifer during the Judeo-Christian Apocalypse, and Cain killing Abel in Genesis. Both are events examined in previous seasons incorporating Sam and Dean as representations of these biblical figures.