Dario Argento films (and giallo films in general) are not for everyone. They exist in their own world. The acting, pacing, and logic can test a viewer’s patience. But their sole purpose is focused on one thing: tension. Every choice made is designed to increase the discomfort of the audience, to build the feeling of unease until the violent payoff. Deep Red sees Argento at his most creatively engaged, with each kill an example of terror-filled build up leading to a terrifying conclusion.
The idea of the killer being inside the entire time is the definition of tension. I’m going with Black Christmas.