It wasn't the first time Illya had blood on his hands. It wasn't even the first time that human blood was on them. It was, however, the first time he had blood on his hands from a man he had killed himself.
He was still so young and small for his age - it never occurred to him that he might kill the soldier. Illya only wanted to make the man stop hurting the girl, so he picked up the rock and struck him in the head.
The girl's sobs broke him from his stupor. They both ran.