The subject of The Hireling Shepherd was actually taken from William Shakespeare's King Lear. In the play, the character Edgar sings a song about a shepherd neglecting its flock, and Hunt used the song as an inspiration for his work.
Hunt actually wrote a letter on this subject that I found on Shakespeare Illustrated-- that connects 19th century paintings to their roots in Shakespeare's work.
Shakespeare's song represents a Shepherd who is neglecting his real duty of guarding the sheep: instead of using his voice in truthfully performing his duty, he is using his "minikin mouth" in some idle way. He was a type thus of other muddle-headed pastors who instead of performing their services to their flock--which is in constant peril--discuss vain questions of no value to any human soul. My fool has found a death's head moth, and this fills his little mind with forebodings of evil and he takes it to an equally sage counsel...