Contrary to popular civilian belief, a shinobi's first kill does not leave him collapsed on the floor sobbing hysterically. No, all too often, that first kill is made unknowingly, a careless slash here, a self-defensive thrust there, before one knows it, they have at least five kills under their belt.
In fact, if you go up to any shinobi and ask how many people's death they have caused, they will not answer you for they do not know themselves. Everything a shinobi does tilts the balance thousands of people depend on. How many villagers have died due to the utter devastation of the land they lived off of? How many heart-broken family members took their own lives after hearing the deaths of their shinobi relatives? How many friends killed on their misguided search for revenge?
No, it is not the first kill that breaks you, it is the first death. When you register the slippery blood flowing through your fingers, when you smell the sweet aroma of rotting carcasses, that is when you break.
For many young genin that realization comes as they clean up after a battle, disposing of the dead, sharpening their weapons, ensuring those who are 'dead' stay dead. It is then they realized that being a ninja does not revolve around saving lives; it revolves around taking them.
During the great war, that realization came all too soon, children running around with kunai, stabbing everyone and anyone in sight. Childhood friends forced to turn on each other.
So no, the first kill makes almost no impact to most shinobi, it is the first death that first cracks a shinobi's psyche. A crack that will always remain.
I'm on a roll :P Three chapters in a day, hope you guys enjoyed it, please review
Disclaimer: I do not own Naruto