On another world, in another time...three men were trapped in a cabin at the mercy of a demon. One of those three, possessed by the demon, was shot in the leg by a gun that could kill anything. The two other men, brothers, attempted to recover from the ordeal only to find the third man - their father - alive and struggling against the monster within.
"Sammy! It’s still alive. It’s inside me, I can feel it. You shoot me. You shoot me! You shoot me in the heart, son! Do it!"
"Sam, don't you do it. Don't you do it!"
"You’ve gotta hurry! I can’t hold onto it much longer! You shoot me, son! Shoot me! Son, I’m begging you! We can end this here and now! Sammy!"
"Sam, no..."
"You can do this! Sammy! Sam..."
In that world, in that time, Sam Winchester did not pull the trigger and shoot John Winchester in the heart. The demon, Azazel, would go on to open the gates of Hell itself. While he would eventually die at the hand of Dean Winchester, the damage was done. The opening of those gates was one of the first steps on the road to the Apocalypse.
But this is not that world, this is not that time...
In this world, we get the answer to the question: What If...Sam Winchester had killed John Winchester?
Let's peer into the looking glass...