1973 saw a severe decline in the popularity of the Giallo and Sergio Martino's Torso, the first Giallo to be released that year, is definitely the harbinger of this shift. Over 45 films that qualify as Gialli were released between 1971 and 1972 compared to only six in 1973.

The most interesting thing about Torso is its clear delineation from a proper Giallo in the first half of the film to a nail biting slasher in the second half, with the turning point being the triple murder of the girls in the country house. Until that point, Jane is clearly defined as the amateur detective, but as soon as her ankle injury renders her helpless, she quickly becomes the object of killer's prolonged stalking in the final act - a formula that would be reused ad infinitum in all the slasher films that followed. 

Gialli employ a certain amount of tension and suspense, but the second half of Torso amplifies this to an almost hyper level. Once Jane discovers the bodies of her friends, the 20 minutes that follow are textbook "cat-and-mouse" slasher film techniques. These scenes are not unlike the home invasion scenes in Alexandre Aja's Haute Tension which clearly draws from Torso for inspiration. 

Eventually however, Martino returns to the well established standards of the Giallo to close the film. The killer is revealed and his trauma-induced motives are explained in classic flashback style. There's even a final showdown between the killer and the token alpha male doctor who credits "providence" (at least in the English version) with Jane's rescue.

Also interesting is the the final scene which is set up to make the viewer think that the killer (not completely dead yet) will attempt one final murderous lunge from the left frame as the couple walks out.

...although maybe I've just been conditioned by too many slasher films.