I'm Your Worst Nightmare
For quickreaver's prompt at ohsam:Pretty much what it says on the tin: if you can't beat 'em, join 'em? Demon!Dean doesn't want Sam dead, not really. So okay, he goes after one-armed Sam with the hammer and maybe ends up catching him and getting a little too, well, ambitious ... Dean has no choice but to feed Sam his blood before Sam dies.
So yes Dean went to far and choked Sam to death... and then he had an idea.
Title taken from Novocaine by Fall Out Boy.