These were celestial medics who performed mercy killings on mortally wounded angels on the battlefield. On Earth, this angel has begun "mercy killing" humans who are experiencing even minor emotional distress.
The narrative follows two parallel storylines: Castiel’s struggle with humanity and Dean’s investigation into a series of mysterious disappearances. [S9E6] Heaven Can't Wait
Castiel’s journey highlights the dignity found in "lowly" work. His pride as a former Seraphim is replaced by a genuine desire to be a "good" human. These were celestial medics who performed mercy killings