Dean could feel them both dying next to him. He could feel their souls leaving their body, just like he could feel his own doing the same. He could feel Cas's hand clenched around his own in a crushing grip, and he could feel it weaken as each second passed agonizingly slow. He could feel Sammy's struggle, and he could hear his gasps for air.
He should have acted faster with the witch. He should have killed her. But it was too late.
Cas was to his right, facing him and staring into his eyes with an expression of calm understanding and those chillingly blue orbs that seemed to grow dim as their final seconds were slipping away. It was agony to see Cas torn up like that, with a long, deep gash going from his left hip to the top right of his torso. But Dean knew he had to stay strong and comfort him in his final moment.
Dean watched as those eyes - the eyes that used to give him chills, that used to tell Dean that he was loved even without either party needing to speak, that used to convey the emotion that Cas never let onto any other feature of his face - became lifeless and dull.
He nodded through his sobs and choked out an "I love you" that was so full of raw emotion and sadness and grief.
Sammy was to his left, with his shirt caked with blood, both dry and wet alike. He was still gasping and coughing and choking on his own blood as Dean turned his head toward him. Their eyes met and Dean felt more helpless than ever as he knew that this was it. This was the final time they would die. There was no going back like all those other times before, and there were no redos.
Pointless protectiveness surged through his veins as he reached toward Sammy with his left hand, pushing it onto the top of his chest. He choked on new sobs as he felt his brother raise both of his own hands to fall on top of Dean's, and it was obvious that the motion alone was too much for Sammy to handle. Neither brother was strong enough to move any more as they choked on sobs and blood.
Sammy died before Dean, and Dean was forced to watch the life leave from his brother's eyes. It was only natural that Dean had to watch the two people he loved most die, because of his own stupid mistake, he figured. There was a soft, broken "Goodnight Sammy" that was whispered before Dean succumbed to the darkness.