Childhood cruelty grows from comparison, not malice
The worst harm in the story does not come from a bad child but from an ordinary one who measures himself against a sibling and keeps finding himself smaller. Each small resentment, such as praise for the brother, the brother's easy kindness, or the girl's attention, is individually trivial, but together they build pressure until one impulsive act releases it. The mechanism is accumulated comparison, not intent, which is why the perpetrator is as shocked as the victim.