Map of your head
If he had ever thought about it (which he was not apt to do) Zuko would have supposed that the first serious injury he dealt would have been in firebending training. Fire was temperamental and, although he was not an introspective person, he knew he had a bit of a temper. He trained with experienced people, and his soldiers were good with releasing just enough fire to cancel his strikes, but accidents happened.
With his swords he had always felt so in control. Blades couldn't run away from you, couldn't take on a life of their own and burn and bite and eat your flesh.
Or so he thought.
If he had paused, in the helter skelter shambles that was his life, to ponder for a moment, he would naturally have assumed that the first person he killed would be an earthbender. The Fire Nation was involved in active warfare against the earthbenders, and they would be the ones most likely to shield the Avatar. If he had been given to mental images, red blood on green cloth would have been what he conjured up.
He believed you only killed your country's enemy.
And if he went too far, there was always Uncle Iroh, ready with a quick step and faster tongue to stop things from going wrong.
So nowhere in his supposed musings did his current scenario exist. His guardian Iroh lying underneath him, hand pressed to his chest, panting wildly; people of all colours surrounding him, blue beside him (backing him up, his mortal foe) green behind him (protecting his back, enemy of his country) yellow above him (giving him cover, the one he wanted dead) and red before him (bleeding on his blades, his kinsmen).
Red on red, his broadswords ran away with him, cutting through the young girl's flesh like a hot knife through butter. The work of an instant, he could not stop until the stroke was finished. The girl he had known since childhood, the one who had gotten too close, had chosen left instead of right (she would have lost a hand that way, but what is a hand to a life?) now gave a gargled choke and, with two wet thumps, fell to the ground.
Horror coursed through him, and for a moment he thought he was going to be violently ill, but then another figure in red raced towards him, trampling through the dead girl's blood, and moved to strike.
And so he swallowed his fear, the sick feeling of his world once again getting yanked from underneath his feet, rejected the urge to shut his eyes for one brief moment in memory of Ty Lee (for that moment would have meant his own death) and instead slid back into position, once again ready to kill.