There has always been much ado about the ending to High Tension. It's one of those topics that really seems to divide people into the requisite love it/hate it camps of life. I can not tell you how many times I have heard that the ending to the movie completely ruined the film for someone. I can sorta see their point, I suppose. Up until the ending, you are watching what amounts to an excellent new age slasher, the likes of which hadn't been seen in years when High Tension came out. Then you get to the end and at first, it feels a little cheap. You feel like you've been tricked and you're left ultimately pondering what the fuck the point of the head scene at the beginning of the film was. Well, I can't answer your questions about that. I can however share my insight into the rest of the film. In case you didn't get it, by now, I am going to have to spoil the hell out of this movie, so please stop now if you've passed on it.
The film basically opens with Marie sitting in a Doctor's office and the Doctor says "Why don't you tell us what happened?" That line right there is the key to unraveling this whole mystery. You see, Marie is what psychologists refer to as "nanners". The entire movie from beginning to end is her twisted recollection of how her fractured psyche perceived the events. I don't care if that isn't how split personalities and dissociative disorders work. If Fight Club can get away with it, so can High Tension. The point is that while most of the events that we are shown do happen, they do not happen the way Marie thinks they did. That is why whenever Marie and Alex are alone together in her room or in the truck, Alex looks absolutely terrified of Marie, instead of having that pleading "save me!" face you'd expect from someone in her situation. It also explains why this adept, meticulous killer could manage to never notice there was a victim-in-the-making within 50 feet of him for half the movie. The truth of the matter is if you take away the twist, the movie REALLY stops making sense.
I do not expect everyone to like the big twist and I completely understand your disappointment, because Phillippe Nahon played an absolutely stunning killer in this. For someone to be that menacing and not have to resort to costumes and parlor trick kills to get there is mighty impressive. So I feel your pain. What bugs me, mostly, is people saying it doesn't make sense. To those folks, I say ye "Watch it, again". Much the way Fight Club seems like it beats you over the head with the truth upon multiple viewings, I feel Aja did all that he could to let us know Marie was our killer without ruining the fun and coming out and saying it. It's not like High Tension pulled what the My Bloody Valentine remake (which was bad ass, by the way) did. Watching it multiple times really lets you see just how obvious it is. From the terror in Alex's eyes to the "You're the only person here, you crazy bitch" look the gas station clerk gives Marie. All the signs were there. They were just subtle enough that you didn't see them if you weren't expecting a twist.