Lt Flynn says the killer brands each victim with a number using Roman numerals. This victim's left heel has "II" (Arabic number "2"). If she was the eleventh victim, the killer should have written "XI" (11).
Brenda is making a perfectly innocent phone call in the maintenance shop and yet the killer miraculously figures out she is on to him with absolutely no clues to warn him. In addition, he somehow manages to get in front of her as undetected where he ambushes her with the cattle prod. It makes no sense other than the fact that it is dramatic..
The cattle prod used on the victims is described as a mild device that does not shock a person like a Taser, yet Brenda is knocked across the room when she is shocked which should not have happened with a prod like that. In addition, she was shocked through two layers of clothes, one of which is an overcoat, so they would have acted as insulators further reducing the severity of the shock. Beyond that, she has to spend days recovering.