Just re-watching the Brothers Keeper and at the very end after Sam lays down the pictures, Death says "It's for family you must proceed, Dean. To be what you are, to become what you've become is a stain on their memory. Do it. Or I will." I don't think that it is really Death, mainly because I don't think he would have said that to Dean. I didn't catch this the first time and I am just now re-watching it. It really seems out of character for him to say it. Who knows.