The act of killing has two consequences, with one victim on either end. If right now your brother is a little bit immortal, only one side matters.
It won't really kill him. Demons don't exsanguinate. But his body will still bleed out, Dean's body will; and part of Sam is sure that when all of this is done and he's won his brother back, Dean's heart will still remember the aggression and it will fester and always, there will be this between them.
Even best case scenario, Sam will remember. It'll be his heart.
It's funny, because the hammer's not at issue, not in the same way. It's a problem, and Sam will deal with it, but that's all it is. If this works, he'll leave Dean to remember the hammer, but its claw, its cheek embedded in the plaster, its head ready to swing back and meet Sam's own, will not haunt Sam.
There's something to that, Sam figures. And it's not because this "isn't" Dean, or it "isn't" a hammer. Right now, though, he has a more immediate problem, the hammer. It's a temporary problem, at least, and this is how he'll deal: He will slit Dean's throat.
He does. Dean drops his chin over the blade and Sam pulls quick, slightly upward--carotids, vocal cords. Dean bleeds in, there's not much mess, he's on the ground unconscious in under five. Sam's always been good with a knife.
Before he dies Dean is voiceless, his vocal cords Sam's most intentional targets. Because Sam will slit Dean's throat, but he won't let Dean say I'm proud of you.
Not for this.