I actually disagree completely. It wasnt obvious at all that it was gonna be her the whole time, mostly because unlike a lot of these types of films that leave little clues or make things vague so they can explain the twist later, High Tension had absolutely nothing suggesting it. In fact, they just straight up showed a completely different character and a completely different chain of events throughout the film than what is ultimately revealed to be what "really happened." So of course none of it makes any sense when that is shown to be the twist, most of what they claim she did is literally impossible and its just really dumb. Its basically like if you were watching Texas Chainsaw Massacre and at the end the movie revealed that Leatherface never actually existed and it was the main teen girl killing her friends the whole time, and they went back and replayed earlier scenes showing said teen girl killing everyone w chainsaws, being in multiple places at once and displaying superhuman strength.
Shame cause it was really well done other than that dumb twist.