Aang couldn't say why he'd decided to take a walk around the zoo---since arriving, he'd seen most of it, after all, and it wasn't as if the animals were terribly entertaining when most of them were asleep. Maybe he'd just known that Leslie would be there, and would need his help.
Whatever it was, he was still barely--barely--in time to save her. The sight of that vampire leaning over her, ready to kill her, scared Aang in a way he thought he'd never experience again. It was like seeing Katara being attacked, or worse, and Aang barely had time to think before he was reacting.
"Get away from her!" he howled, and his voice was a gale-wind, his hands bending the air into a blade that screamed through the air---slicing the arm holding Leslie right off. Another swirl of wind broke off from the blade, turning gentler as it picked the girl up and pulled her backwards, toward the Avatar. --He stepped immediately in front of her, lips drawn back in a blunt-toothed snarl, eyes luminous as he fought down the Avatar State.
"Leslie--" he addressed without glancing back at her, gaze fixed on the vampire, "are you okay?"