Id call it a thriller. I dont consider it a “horror” film really. It has more in common with the Italian giallo films of the 70s than the 80s slasher films. Its not like the Friday The 13ths where you see people getting hacked up for 2 hours. Theres really only one kill sequence in the entire film and its not even that gory.
What QT did was make the film using cinematic themes and language that originated in the gialli & slasher films. What it ended up being in the end was something else thats really undefinable. Only QT couldve made Death Proof.
