mary appears to sam as unconditional love and acceptance and comfort. what better figure for sam’s mind to use for comfort and to validate himself than to use his mother’s image? he takes comfort in what she could have been like, what he’s heard from dean and john. he’s crafted an image of her from what he’s heard of mary through the years and is using it as his own safety blanket. he needs to feel acceptance and approval of what he’s done to know he’s not a monster or a freak and what he’s worked for was for a good/greater cause, and mary is that comfort he wants. this scene will never not fuck me up