The Judas Kiss
I'm your new God.
Those are the words that haunt him. It was in the moment he'd heard them coming calmly out of Cas's lips, yet strangely it wasn't Cas speaking them, that he'd realised his friend was gone. The shred of hope that he'd been clinging onto, that they could somehow still work this out, was obliterated with that single, chilling sentence. Dean can barely stand to think of it, as he remembers all that happened. Castiel, the angel who was once his friend, but then betrayed him. The angel who went behind his back to ally himself with Crowley, but then who, like Judas, had to pay the price.
The image still lingers in Dean's mind, of a trenchcoated figure wandering forlornly into that reservoir and then slowly sinking beneath its surface, vanishing to him forever. Maybe Cas brought about his own destruction, maybe he even deserved it after everything he'd done, but Dean still can't shake the sense of loss. There's anger too, at how Cas had betrayed and hurt him so deeply, but more than anything there's just an emptiness. It feels like all the rage is just a cyclone whirling around an empty space where there's nothing left to be angry at anymore, and no matter what happens, it won't ever be filled.
Alcohol provides some small comfort, as he let's its numbness wash over him like the black water did over the angel, but he knows that no matter how many bottles he downs or how many Leviathan heads he hacks off in revenge, he's lost something he can't get back.