"Get out, get out, GET OUT!" Sam screamed. Sam had always been volatile, but Dean was getting worried. Sam was scratching himself, mangling his flesh with his own fingernails.
"Sam, stop!" Dean pleaded. He never knew what to do when Sam got like this and it scared him.
"No! Not until you get out of my skin. Not until I can't feel you on my body anymore!" Sam continued scratching at his skin, as if the pain would wash away the memory of what they had done.
This is why Dean rarely has sex with Sam, because this always happens after- although never before to this extent. But sometimes Sam asks, begs, says he needs it, to remember they're still alive, that they're still together. To join them permanently. And Dean can't say no.
And so, as punishment for his crimes, Dean stands there and watches Sam destroy himself. Dean can think of no truer justice than that.