I feel like they were more shocked by the fact that Angela was really Peter than by, I don’t know, the severed head Angela/Peter was holding. The movie also gives no justification for Angela/Peter being evil besides the fact that Peter was forced to live as a girl. While, yes, it would be traumatizing to be raised as a different gender from the one you identify with (which of course is what trans kids face), the film suggests it would turn people into unbalanced, bloodthirsty murderers.
