Brody's arc had to come to full circle. In Season 1, he returned to the states as a hero despite being on a suicide mission for Abu Nazir. He ultimately failed his mission, remained alive and turned to the CIA. Eventually, in Season 3 he went to Iran with a similar mission and this time, he succeeded, but had to pay with his life. Essentially, the last few episodes of Season 3 are the mirror to Season 1.