You don't know the true meaning of "Where did it all go wrong?" until you are tied to a chair with a dirty rag stuffed down your throat and a gun to the back of your head. At that point you really start to re-evaluate your whole life. Dean isn't struggling against his confines like the blue-eyed angel is. Castiel is thrashing against his ropes, pulling on the ones around his wrists so tightly they're beginning to bleed. Dean wishes he could tell him to stop trying to escape; there isn't much of a use. Dean just sits still; quiet, patient. He sees the empty warehouse around them, the angel sigils on the walls, the ceiling. They're draining Castiel's powers, killing him slowly. Castiel is panicking, his eyes wide and frightful. Dean does his best to smile around the cloth is his mouth. He does his best to console his angel without words; to tell him, Everything's okay. And, I forgive you.
Dean has made a lot of mistakes in his lifetime. There aren't many choices that he's proud he's made. But he is proud that he made Sam stay at the hotel. He is proud that he unknowingly saved his little brother. He is also proud to die alongside the bravest soldier he had ever met. It was an honor to die for Castiel.
Dean could see Castiel's mouth moving frantically. He was yelling, but Dean couldn't hear the words. He wasn't sure why that was. Perhaps it was the multiple blows to the head he'd received. Or maybe in the adrenaline of being trapped his senses had faded. Dean didn't honestly care. Though Castiel's eyes were locked on Dean's as he shouted, Dean knew the words weren't aimed at him. They were aimed at the man behind Dean, the gunman. Dean felt the man pull back the hammer on the pistol, and suddenly his senses came rushing back to him. He could smell the stale air in the empty room, taste the grease on the rag. Dean could feel the sweat along his skin, sticky and damp. He could feel ropes cutting into his flesh, rubbing it raw.
And Dean could hear Castiel's screams. They were pleading, panicked. He screamed, "No,"and "Stop,"and words in languages that Dean didn't even recognize. Curses and prayers and promises. Dean's heart dropped a bit at the sound; such a beautiful creature should not be allowed to hurt that badly. Dean focused his eyes straight on Castiel's. He hoped Castiel could see the forgiveness in his gaze, hoped he understood that Dean did not blame him for this.
Dean shut his eyes just before the trigger was pulled. The last thing he saw was wide blue eyes.
The last thing he heard was Castiel's pleading voice. Though he cried a single syllable, Dean heard a thousand words. He heard: I'm sorry. And: Forgive me.
I tried so hard.
And: I'd do anything for this to be different.
I will find you.
Dean heard: I love you.
But all the gunman heard was, "Dean."