God's Angel commands Eliyahu to confront Ahazya's messengers, instead of speaking with Ahazya directly. An explicit message is conveyed to Ahazya, and an indirect message for the messengers themselves. The messengers return to Ahazya before completing their mission, and deliver the words God conveyed through Eliyahu to Ahazya. Their actions prove their loyalty to Eliyahu and the word of God, and repentance for their prior loyalty to Ahazya. 

Courtesy of the Virtual Beit Midrash, Yeshivat Har Etzion