High Tension is a classic of New French Extremity horror, and often cited as one of the greatest horror movies of the 21st Century. I stumbled upon an interesting reading of High Tension on movie critic site Bitch Flicks, which eloquently defends the movie from several common complaints. I'd say they hit the nail (or should that be cupboard?) on the head...
On the controversial twist:
When Marie ends the killer’s life, it is revealed that she is in fact the killer, thereby rupturing the classical protagonist/antagonist relationship.
Aja’s ending has received strong, negative criticism for its twist, but the purpose of this ending is to not merely shock. Of course, if we read it through a conservative lens, then Marie’s transgressions serve to maintain and perpetuate heterosexist discourse, as the lesbian protagonist is revealed to be the monster; she is the outsider who has destroyed the nuclear family.
On representation of queer violence:
It could be argued that this (fe)male violence symbolises Marie’s anger, or more specifically, Marie’s inability to control the rage she feels about heteronormativity upholding “traditional family values” (these being strictly defined gender roles and heterosexuality). After all, she cannot control this part of her consciousness, as she desires to kill this part of her consciousness and rescue Alex.
On the gender binary:
High Tension is aware of its supposed inconsistencies, which again can be seen in its ending. Before Le Tueur’s death, he wields his chainsaw in an attempt to kill Alex, only for his weapon to be replaced by Marie’s sweet and soft kiss. The act of (fe)male violence and gentleness in this scene unifies the binaries of masculinity and femininity, and therefore complicates the definitions of monstrousness and gender.
High Tension offers no concrete resolution as to how we should view the protagonist. Instead, it offers multiple readings of gender, sexuality, and violence that typify our contemporary, heterogeneous culture. Indeed despite the monstrous actions of Marie, underneath the surface, Alex and the audience know that she cannot be simplistically defined – it is why we have returned to her at the hospital.