But why must Allah judge the dead?
My assumption will be, the dead seek/ask for Justice. So to give Justice, Allah must judge the dead. Therefore it is not the will of Allah to judge, it is because the dead ask for it.
What's your opinion?
No, not like that.
Allah SWT asked all of His creations, such as angels, mountains, planets, stars, etc., if they wanted a free-will and be held responsible for what they do in the universe. All entities in His creation declined....except we foolish humans.
To decline & to except is free-will
If Allah asked all his creations, then all his creations already had free-will.