Aberforth blamed his brother Albus because the tragic event occurred during a three-way duel involving Aberforth, Albus, and Gellert Grindelwald. Ariana, whom Aberforth and his mother had spent years caring for, tried to intervene in the fight and was hit by a stray curse, which resulted in her death. Aberforth held Albus responsible, believing that his brother's obsession with Grindelwald and his neglect of family responsibilities led to the fatal incident. This caused a deep and lasting rift between the Dumbledore brothers.