"Nothing is more wretched than the mind of a man conscious of guilt." - Plautus
Castiel only happened to be there when the decision was made.
When Bobby chose to go.
He was returning from an Amsterdam meadow, his mind still scattered enough for him to not know why he wanted to return so soon. He just did.
As he made his way down the cabin stairs, he froze at Bobby's words "I'm done".
The Winchesters did not see Castiel at the top of the stairway as they prepared to send Bobby off, and neither did the spirit himself.
Castiel observed them, suddenly the most sober he'd been since he'd taken on Sam's broken wall and the memories that had lain behind it.
He eventually sat on the stairs, forlorn, as Dean took the flask out of his jacket, memories of the Apocalypse and the times he had spent around Bobby flowing though his mind.
They hadn't always gotten along, but they had been a part of "Team Free Will" as Dean had called it, and Castiel had grown to like and respect the gruff old hunter the Winchesters had come to love like a father.
To know that the leviathan leader had been the one that shot Bobby and ended his life was to know tremendous guilt. He was responsible for what the leviathans had done to humankind; for the sorrow written on Dean and Sam's faces; for this moment being Team Free Will's last.
For this silent goodbye.
I am sorry, Castiel thought for the thousandth time, as Bobby lit up and disappeared.
In the darkness after the light, Castiel watched Sam and Dean as they mourned in silence from his position on the stairway.
Knowing, painfully, that apologizing would never be enough, and yet, that was all he could do.