I have considered it as a horror film personally, it is subjective though and down to the viewer. Considering the time it was filmed it's definitely got horror connotations but I feel that if it were a film that came out today it would just be classed under the very wide umbrella of "psychological thriller".
At the end of the day though, you only need to be able to broadly categorise something with a genre so you know quickly if it sparks your interest. A good film is a good film, and a bad film is sometimes a good film but it matters not whether it's horror, drama or thriller.
