Aang's eyes narrowed on the figure cloaked in shadows before him, a muscled arm wrapped around Katara's waist and a glinting blade to her throat. The ultimatum was simple: renounce his duty as Avatar and never again attack the Fire Nation or the Fire Lord. He had mere moments to respond in the affirmative before Katara was killed in front of him.
Aang met Katara's eyes for the first time since this all started and spotted the fear beneath her bravado. He could not – would not – let her die. Not like this and not when he knew he could save her. Taking a deep breath, Aang felt himself falling into the practiced mindset he hadn't had to in over a hundred years. Something settled in him, cooled and hardened, and he thought Katara must have seen some shift in his face, his eyes, because she looked horrified now rather than terrified.
Still and steady as stone, as thick ice, as a wall of wind in a hurricane or a caged flame, Aang caught the threat's black eyes and held them, not turning away even as he squeezed his hand into a fist by his thigh. He watched with no expression as the life drained from those eyes, the air forcefully expelled from every corner of those fragile mortal lungs.
The knife fell soundlessly to land edge-first in the earth. Katara was released as the arm holding her went limp, but she didn't move, just stared at him like she had never seen him before. Aang didn't even acknowledge her. He shut his eyes as soon as the threat was neutralized and took another deep breath – it was always harder to release the killer's mindset afterwards, but he had plenty of practice.
Air bending was a passive, defensive art. Everybody knew that. But even airbenders understood that death was not always avoidable, mercy not always feasible, and Aang wasn't just an airbender. He was an Avatar.