The end of the world just happens.Well, something like that. Sam isn't really very keen on where it began, other than when the Croatoan virus suddenly hit big in parts of the United States and burned outward in every direction; at this point, Sam had already said his goodbyes to Dean, had picked a hemisphere (non-literally, because he's not so sure he could forge plane tickets to China right this moment), started to try to correct his life. There were dreams, yeah, some Lucifer here and there to intrude on his privacy. But he was managing. Until, y'know. Shit hit the fan. In the end, phones went down, electricity, running water supplies. Places shut down. Boarded up. Humanity, trying to thrive. Lucifer, celebrating in his nightmares.
Sam never did speak to Dean again, after their final goodbye. Now he doesn't know if his brother is alive or dead out there.
Maybe he'll never know 'til it's too late. Or until he's dead. Sam's a pretty lucky guy, all things... considered. He's immune. Been bitten more than once even (covered by his jacket, because if people saw... well, he's not about to get mistaken for infected, even if they're only scar tissue now), though most of it was born out of a sick sense of penance. He's hardly afraid to go barreling into a horde of insane viral carriers if it means maybe saving one life who can't handle getting infected. He can go on supply runs for people, too. The more the world eroded away, the more he finds himself wandering from place to place, directionless and distraught.
This is all his fault. It's all on him.
He should have never let the devil out.
He should have never believed a demon could be anything but a twisted, ugly demon.
He should have... Should have done something different.
Bobby isn't in his home, Jo and Ellen are MIA, no signs of life from anyone he knows. No Dean. Just more people to help. Mouths to feed. Croats to kill. By the time he reaches a little place in California, he's exhausted, seeking out an abandoned building to rest up there. He uses his pack as a pillow and listens for any sounds of the infected. Or survivors. It's not like he can deny someone in need of help, in a mess he made. There's probably some big issue just around the corner. Danger. Right now, his only concern is actually getting a few hours of rest before the Devil comes to poke around inside his noggin.