He was trying to capture Aang himself to regain his honour. But if Zhao presented the Avatar to the Fire Lord, Zuko would have lost his chance. So he had to steal Aang from Zhao, but he was going to take Aang back to the Fire Nation as well.
Stealing another's thunder is pretty common between allied organizations and even people within those organizations. Hence, jurisdictional disputes.
Edited on 10/13/2006 4:07pm