Guns don't have much in common with Amazonian weaponry, in general. There's really no point, prior to the German invasion, in which we see people native to Themyscira have any experience with a gun. We certainly see them use ranged weapons — Wonder Woman 1984 even opens its narrative with a flashback on Themyscira, wherein we see Diana very confidently shooting a bow and arrow, and hurling spears. Guns, though? They just aren't a part of Amazon artillery.
Unfortunately for the Amazons, when the Germans do invade in the first Wonder Woman movie, they absolutely do have guns. Keep in mind, Diana is the only child on Themyscira, so she's kind of every Amazon's kid, but it's made clear that she has a special relationship with Antiope (Robin Wright), the strongest fighter on the whole dang island.
So, then, imagine the sense of unfairness that Diana feels when her aunt dies at the end of a German rifle. Remember that the Amazons, up until that point, have effectively been immortals. That means Diana, throughout her life, has never really known death until Antiope is murdered by a gun.
That factor, alone, is enough of a reason for Diana to hate guns. And she wouldn't be the only DC superhero who has such negative associations with firearms, either, which might play into her later connection with Batman.