After Ziel heard Azalia's question, he realized that he had never thought of that possibility until now. He was sure that while fighting Azael, they intended to kill each other. But in the end, Azael cast his curse by sacrificing his life force on Ziel instead of dragging Ziel to death with him.
Ziel finally thought of the strangeness that Azalia felt about the story. But no matter how much he thought about it, he couldn't find a reason for Azael's actions back then.
"I do not know. It's probably only Azael who knows the answer." Ziel sighed heavily and tried to put the matter aside for now.
"I understand, and I trust you. I'm sure my brother has other goals in keeping you alive, Ken." Azalia removed the veil covering her face to show her trust in him.