Sam: whispering, screaming
I could write a million words about this scene, mostly about how fascinated I am by the apparitions Sam’s subconscious chose to represent his torment in this moment. (I would have stuck this under a cut but Tumblr wouldn’t let me.)
Alastair: Sam’s torturer, representing pain, fear, consequence and (probably) death. To me, this is the most fascinating ghost. Sam had defeated Alastair - he’d proved himself stronger, yet it is Alastair who returns to torture him: not Lucifer, not Lilith. Alastair, the demon who broke Dean and “carved him into a new animal”. This is the creature Sam still fears. Because if he can break Dean, what chance does Sam have? For all Sam’s power, he still measures himself against his brother, no matter how hard he tries to set himself apart.
Young Sam: the most truthful of all Sam’s ghosts, representing honesty, anger, regret. He’s the only ghost to whom Sam really tells the truth - and he looks him in the eye while doing so. He can face his past, accept his choices, tell his younger self that he has put away childish things and the real world is never going to allow a Winchester a normal life. Which breaks my heart into a thousand pieces. Young Sam is Sam’s lost hopes and dreams. The him he thought he could be; the happiness and normality he will never have.
Mary: even covered in blood, it’s Mary who offers Sam comfort, reassurance and validation. In his idealised imagining of her, she is the one who offers him hope that he’s doing the right thing; that he’s strong, righteous and brave. He doesn’t imagine his father or brother or any other authority figure who has been present during his life. Nor does he imagine Ruby, the only person in his real life who is actively supporting him and telling him he’s doing the right thing - only his dream of a mother’s love can give him comfort.
Dean: during his life Sam experienced many things with his father and most of them weren’t good - disappointment, rejection, a sense of alienation. But it isn’t his father who represents those things now. (I realise that may have more to do with JDM’s availability but WTF it’s canon now, so…) It’s Dean. Not John. Not Bobby. Not God or the angels. Dean. Dean, whose approval he yearns for more than anything else - and who rejects him. Sam could bear turning his back on his own dreams, everything he ever wanted in an idealised version of his life, as long as he had Dean in his corner. Dean’s rejection is the ultimate admission that somewhere, deep down, Sam knows his path is dark, dangerous and likely to end in tragedy for everyone. And it’s a path he’s going to walk alone.