There are two kinds of people when it comes to horses: horse lovers & the other kind.
If your aunt is a horse lover, she'll know that it's never, ever the horse's fault. (Her accusing the horse of being dangerous is a clue that she's not a horse lover, especially in light of the fact that you get along fine with the horse.)
If she's a horse lover, she'll take on this horse as a challenge to her savvy & at least learn to get along with it in leading it from point A to point B.
The horse absolutely senses her fear, which tells him that he must take charge of her, since she's too scared to be the leader. (In horseville, SOMEone has to lead - equal division of power doesn't work).
If she cares, she will gain the skills & confidence to be his leader, because she acknowledges that he's not at fault. She'll do that by developing herself with easier horses, getting a mentor, getting a home study course like Parelli Levels, which go into adult learning & fear issues, or some combination thereof.
I suggest that you present her gaining some savvy in a positive light, because it really IS positive to overcome your fears & enjoy a horse rather than blame it & send it down the road. You can tell her that fear is normal, no shame in the brain trying to keep one alive, but that it's a real high to gain the skills to move past fear.
If she's uninterested in developing any capacity with the horse, I suggest that you try & get her to admit that the horse isn't dangerous, but that she simply doesn't want to gain the savvy. Then the blame game on the horse will be finished.
Hope you get to keep him!