One summer, Albus was about 17 or 16 at the time and his brother, Aberforth was mad at Albus. He was upset that Albus was going to Hogwarts and not taking care of Ariana. Albus's plans at the time were going to be to run off with Grindelwald and together they would become powerful. Albus was sick of this and the argument became into a wand duel. Grindelwald (who was there at the time) even got involved. Ariana (who is a Squib) got involved. She didn't want her siblings to fight. And when Ariana ran out to stop them, one of the three boys accidentally ended up killing her.

Who do i think killed her? I don't know. One side of me is leaning towards Albus. But also later on, Grindelwald became a Dark wizard. So i feel like it was him.

Next chapter, Continuing What He Did After Hogwarts