Before dying, John tells Dean, if he can't save Sam, he'll have to kill him, should he become evil. The fourth season episode "On the Head of a Pin" reveals that the demon Alastair tortured John in Hell for over a century, with John refusing the demon's offer to stop if he himself would torture someone else.
In Castiel's final moments, he smiles through his tears as the Empty takes both him and Billie, saving Dean. Castiel's final words are "Goodbye, Dean."