I think I have it. Sam finds Dean near death after a futile attempt to kill Metatron. He holds his brother and weeps on his shoulder as Dean admits he was wrong and dies. Sam begs for help, saying he'd do anything to save his brother. Dean's eyes open, and of course, he is a demon. Metatron pops into the frame, claps his hands and says, "Oh, goody!" as the season ends.
The season comes full circle; everyone's had pay back for mistakes they've made. It's the worst possible outcome-a typical Supernatural finale.