Zuko was hunting the Avatar down, he didn't someone else was in the ice that day. That was you, you were Aang's older sister and you had sworn to protect him.

Zuko hadn't known you and you hadn't found Aang until recently so you didn't know him either. You two had met, realising both you wanted to find Aang, so you decided to do it together. You thought he must be Aang's friend, and he thought you were trying to capture him. Eventually, you found Aang and both had opposite reactions.