So Mary Winchester right. When the show first opens up in Lawrence, Kansas 22 years ago, we don't know much about Mary. We learn through the series that Mary was born and raised a hunter, and that she was a damn good one too, but that she hated the life. She wanted nothing more than to run away with John and live a normal, non-hunter's life. She wanted so desperately to forget her past and move on. Then Azazel came along, killed her parents and John and gave her the perfect opportunity to leave her old life behind.
And as far as we know, she does. She abandons hunting completely. She doesn't line the walls or ceilings with demon traps, doesn't keep a silver blade or holy water or silver bullets tucked away in a drawer just in case. The only salt in the house is what she uses for cooking and she doesn't bother to keep track of what's iron and what isn't. In fact, she buries her past so completely that she allows herself to forget the promise she made in exchange for John's life a decade ago, the permission she gave that strange demon to visit her child, and his request not to be bothered.
So I can't help but wonder what went through her mind that night when she left her bed to check on Sam. She clearly suspected nothing at first. The distorted static on the baby monitor was meaningless (though anyone that's ever used a baby monitor would hardly find that strange). She was happy to believe it was John's silhouette in front of Sam's crib, happy to leave the two of them alone.
When she saw the light flickering at the end of the hallway, did it spark anything? Did she think anything about the literal ghosts of her adolescence? Did she wish for even a brief second that she was armed again, or did she curse her old instincts that refused to fail her completely even after a decade's worth of disuse?
Then the sounds of the television drew her downstairs and she saw John slumped over in the chair and that was when the realization finally hit her. That was when she realized that the man she saw in Sam's bedroom was not her husband, but even then, did she remember the demon deal? She was terrified for her baby, as any mother would be, but did she remember at that point the demon that had coerced her permission by killing John one decade before, or did she think it was just some random stranger? As she ran up the stairs, did she wish for holy water, for a gun? What was she thinking as she ran into Sam's bedroom, armed only with her wit and a mother's terror?
Did she regret, in her last moments, ever leaving behind the life? Did she wonder with her final breaths "if only I'd had a weapon?" Did she think where she might be or what might have become of her had she only stayed in peak form and had been ready for the evil that had invaded her home that night?
Did she spare a thought for regret or... did she spend her last moments thinking only of her family, wishing for their safety and happiness in the life she then knew they would need to live without her?