I think you mis-read into that whole scene. Diana isn't wanting to kill the boar because it's ugly (I'm assuming this from your comment about seeing past appearances). Diana is filled with rage that an animal (it could have been a tiger cub or whatever) has fatally hurt a deer (buck, actually), something she didn't want to kill in the first place. Instead of dying quickly, like the buck would have if she had shot it or reached it before the boar did, she got to see this majestic animal bloodied and in obvious pain. That "brute" could have been a beautiful jungle cat or a teddy bear and because of the way it attached the deer and her young heart having been broken at the bloody scene, she would have reacted the same way. It was all about raw emotion and misplaced rage at nature being itself versus how we'd like to think how animals die, peacefully and without suffering.
Diana is a child in this flashback, just because she's an amazon and will some day know better doesn't mean she wouldn't react like I think a lot of children would (pre-teens, teens, whatever). I know I did when I first went hunting and I was 16 at the time. I /knew/ better, but I still flew into a rage when I saw the hunting dog take down the pheasant. I wanted to turn my rifle on the dog and shoot it in the stomach so it suffered how I imagined the pheasant suffered. I know how my meat gets to my plate. I don't doubt for a moment that Diana knows how her meat gets to her plate. Knowing it and seeing the actual hunt (or a hunt that goes badly) are two very different things.