As Katara stood there, in the caves of Ba Sing Se, touching the Fire Nation prince's face with her Spirit Water dangling in her other hand, she could feel her heart softening towards him. Though those seconds seemed like hours, as she looked up at his face, his eyes closed as he leaned into her touch, she felt like a new bond was forming between them. She didn't know exactly what kind of bond, and before she could figure it out, she was interrupted by the wall bursting open.
Everything erupted into a battle, Aang in the Avatar State as he fought Azula and her cronies, and, quite upsettingly, Zuko. Had she not gotten through to him? But she had to defend Aang, to protect him. Unfortunately, that's not what happened, and he was struck while stuck in the Avatar State, and she made the quickest escape she could with his lifeless body in tow. That was where she was going to use the Spirit Water, not for a petty prince who wanted nothing but to end or capture the Avatar.
That was all she was thinking about when, months later, he showed up at one of the group's campsites, asking to join them to stop his father, the Firelord. Once he helped defeat an assassin that he sent to kill the Avatar or something along those lines, Katara, with her hardened soul and perspective on the world, went to his room and leaned against the doorframe, seeming to surprise him. She looked up, her arms crossed and her eyebrows furrowed, her lips forming a hard line.
"You might have everyone else here buying your...transformation," she said, a warning tone in her voice. She had seen the small smile on his face as he stood to greet her, but she wasn't having any of it.
"But you and I both know that you've struggled with doing the right thing in the past," she continued, starting to walk towards him. "So let me tell you something right now. You make one step backward, one slipup, give me one reason to think you might hurt Aang, and you won't have to worry about your destiny anymore."
She stared him down, fury burning in her chest as she stared at his shocked face. After having saved Aang once from the brink of death, she couldn't do so again. She had nothing else, nothing that could save anyone. Even her healing powers were almost no match to the properties of what that Spirit Water had.
Without skipping a beat, she continued.
"Because I'll make sure that your destiny ends, right then and there," she threatened, her eyes narrowing further. "Permanently."
With that, she turned around and walked out, without even letting Zuko say anything. She slammed the door shut and never regretted what she said then, until things started to change, until things became different.