What action did Brando take at the Witch's house the night of the attempted robbery that injured the Witch significantly?

The narrator reveals that Brando did not mean to hit the Witch with Munra's crutch as forcefully as he did on the night of the robbery. He also regrets the fact that his blow landed so squarely on the Witch's skull, a fact that became apparent when brain matter began to leak out of the Witch's head once she was lying prostrate on the ground.