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.
These are the endings played out in Sam's visions.
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.
- Episode: 15.05 Proverbs 17:3
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 of Cain 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
- 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.