Just a load of Harry Potter fan theory's that and I wrote when I was bored I don't know how many there will be, but there might be a bit of a gap between each chapter.
Argument 1. Albus and Gellert have some of the best wand control the wizard of world has ever seen, Aberforth doesn't and was an inexperienced dueller.
Argument 2. Albus more likely than not was only shooting defensive spells to protect his siblings.
Argument 3. Aberforth poked on when the duel was over. The duel had finished but aberforth hadn't had enough and shot another curse which is why gellert use the cruciatius curse on him.
The picture at the top of the page is the original photo from the book in the deathly hallows that told Harry about the greater good. Toby regbo as Albus dumbledore and Jamie Campbell bower as gellert grindelwald and I'm not quite sure who the man in the back is I think it might be aberforth but I don't know who plays him. The original photo is so much scarier than the one the ended up using in post:
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