Do it, you say. You push into Sam's blade, feel its edge against your throat, not breaking the skin yet but oh so close, and it reminds you, that moment before the cutting begins, oh yes, it reminds you, and there's a half second of regret, a half second to think of what a team you could have made, under different circumstances. You push harder against the blade. It's all you. And it is all Sam, and that's why you know exactly how it's going to play out.
Sam stares at you for a second, silently (uselessly) pleading with the puppy dog eyes, then his hand falls to his side. As you knew it would. When you advance toward him, your eyes flashing black, his hands go up in a defensive pose. Dean, he says, all sincerity, all of his completely useless sincerity. I know you're in there. You don't have to do this.
You know you don't have to. You want to. Because what Sam doesn't realize is, whatever little bit of old Dean that's still in here, it's not fighting. It's hiding. It wouldn't want to stop you, even if it could. Sam stammers out a plea as you reach out to retrieve the hammer, still embedded in the wall. With one fluid motion you pull it from the fractured plaster and swing it at the side of Sam's head. Whatever he was saying (please or don't or something else equally unimportant) is lost as his head snaps to the side with the satisfying thunk of metal on bone, of brother on floor, and he's sprawled at your feet.
Easy peasy. You lift the hammer to deliver the coup de grace, but it's suddenly wrenched from your grip, and as you try to whirl around, a pair of angelic arms trap you, gripping you tight.
When you wake up, you're sprawled on the floor of the dungeon, and as the fuzz clears from your brain you're punched in the chest by a sudden memory of someone else sprawled on the floor and oh god, oh god, no. You sit up so quickly that the room starts spinning, but the blurry tan and black blob in front of you resolves into Castiel. And you need to ask him what you've done, but you're too afraid of the answer, so you sit there blinking at him and finally you manage to swallow the heart that has crawled up into your throat and ask where is Sam?
I'm right here, says a soft familiar voice next to you. Sam is stained with spattered blood but seems otherwise unhurt. You reach out to move his hair, to see where his skull should be fractured, where he should be bleeding, and you realize he is holding himself completely motionless, in the way a rabbit would when it's hiding from a wolf, or the way Sam would if a madman was holding a knife to his throat, and oh, yeah, that. Scratch that; reverse. You lower your hand. He hands you a metal flask and you drink from it, expecting the searing burn you deserve, but there's nothing but tepid, metallic-tasting water.
You collapse into the chair, the same chair you were tied to earlier, when Sam was trying to tie you again to everything you'd left behind, and it's appropriate, because he did it; you are tied down to all of it again.
And Sam and Cas are smiling and telling you it's okay now, but you know it's not, because Cas may have been able to heal Sam's fractured skull, but you've done so many things that can't be undone.
You lower the blade. It's going to end here, one way or another. For a second, you think Dean's going to back off, that all he needed was to prove you wouldn't kill him, to rub it in your face that you're completely and utterly powerless here. Then his eyes go black and his face becomes a mask of rage and he moves forward, and almost without thinking, you drop to the floor. You roll to the left and stab upward, plunging the knife into the back of Dean's knee. Dean crumples, roaring in pain and fury, ripping the knife out of your grasp. As he writhes on the floor, trying to pull the knife from his leg, you scramble to the wall and yank down the hammer. You straddle Dean and hold the handle of the hammer against his throat, your injured shoulder screaming as you press down until Dean's breathing is reduced to ragged gasps. Dean is still glaring at you, his eyes still black emotionless spots in his face, when you lift the hammer and swing the handle into his jaw, knocking him unconscious. And as you gasp for breath and clutch your shoulder, you try not to think about how your brother could be human enough to walk out a devil's trap, and still want to kill you. You try very hard not to think about that.