Please tell me why because I have seen other movies with a similar format and they at least earned the killer final girl some kind of anti-hero status that made the big reveal a satisfying payoff.
You don’t have to root for her if you don’t feel like she is rootable-for.
I wouldn’t classify her as a final girl, she was the villain. The film is purposefully playing with our preconceptions and misleading us as to who is actually the bad “guy” here.
So, as you’re watching it, going on the information you have at that moment, you go from rooting for a vulnerable little blond lady, being chased by a crazy-nutso coked-up Kyle Gallner, to like… not, cuz she killed a bunch of people, and like tortured him and stuff…
Plus, I don’t think the film is telling us how to feel about any of these people, sometimes we kinda have work it out ourselves.
The normal reaction could be “finally, the bitch is done!”.
I suppose it could also be “ how sad, then end of the infamous electric girl!”.
Personally, I just enjoyed her acting in that final scene and the way it was shot. A very powerful and appropriate end to a roller coaster ride and filled with artistic merit.