What a horrible?

Where it became wrong? An 8 year old child has killed a 5 year old child. The reason was so small but the result has ended with the loss of a child’s life. While playing, the two children had got a fight for plastic ball. At the end the 8 year old child has stabbed the 5 year old child with a kitchen knife. The knife has spoiled one of the veins in the neck which has led to excessive bleeding. At last the child was dead. Whom to blame? Whether the parents or the society.