Pentagrams can be disturbing, depending on how they're used. When they're carved into someone's back, that clearly falls into the disturbing category. As part of the preparation for the sacrifice she's expected to make, the symbol is carved into Mae's back and is still openly bleeding as she runs away. It eventually heals, but it's probably always going to be a part of her as a reminder of what almost happened.
Just the introduction of the scar is startling. The audience doesn't initially see what it is, but Suzanne's reaction says everything we need to know. When it's finally shown, it's a lot to take in. It's large, all-encompassing, and difficult to hide unless Mae always wears a turtleneck. What's equally as concerning is how her peers at school react. They seem to think it's "cool," and Mae's horrifying disfigurement gives her instant street cred. People call her brave for showing it off with a low-backed dress at the Harvest Dance.
The truly disturbing part of her scar is that anyone who sees it can theoretically recognize where she came from and use the practices of the cult against her to try and force her to return. It is a constant and literal target on her back that she can't escape and would probably take several surgeries to remove.
