As a person who likes to save people Naruto probably won't take his first kill easily, no matter how he acts otherwise, and the only person who can help him bear it will be Iruka. Kakashi knows that.
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First Time
Just as he managed to settle all the rowdy children down in order to teach them some history there was a disturbance. One that showed up in the form of Hatake Kakashi appearing next to Iruka and making him jump.
"Enter through the door!" Iruka yelled enraged as the children all began to chatter and bounce around again.
"Naruto's headed this way. We just got back from the mission."
Iruka paused and asked, "What happened?"
"He killed his first man. I'll watch the brats, he needs you right now."
"Thank you," he said expressively and ran to meet Naruto halfway.