"My dreams aren't as empty as my conscience seems to be" – The Who.
Dean was cursed, maybe he had always been. Maybe the mark only enhanced the misery that was already inside him. His conscience had never been clear; there was ocean of blood on his hands. Blood of monsters, of his victims.
That was not why he felt guilt, or couldn't sleep at night. No, it was not those he killed that haunted his dreams, It was those lives he unintentionally destroyed. People such as his mom, his dad, Jo, Ellen, Bobby, Kevin and Charlie. Especially Charlie who they, he dragged into this life to be slaughtered. It didn't matter that it was not his hand on the trigger, he killed every single one of them. He was cursed. He was beginning to think that Mark of Cain had nothing to do with it, that he was the real problem.
He knew he wasn't worth it, wasn't worth saving. Besides there was nothing left to save, he had died inside a long time ago. For some reason Sam still believed in him, "we are doing this because we loved you." Love, a word he never understood and never felt for anyone that wasn't family. Family, there was a word he knew too well.
He would be damned (as if he wasn't already) if he would let Cas and Sam fall victim to his curse. He would rather die than let that happen. That is why he summoned Death and made a deal.