The brilliance of Hitchcock was that he was able to bring murder, suspense, and mystery to the level of the common man. Here, an average middle class family becomes the center of attention as one of its members is suspected to be a serial killer on the run. Keeping up with his ingenious ways of story telling, Hitchcock moves the focus of the investigation away from the Police and puts it in the hands of a niece (Teresa Wright) who suspects her visiting uncle (Joseph Cotten), whom till recently she adored, to be the serial killer.
Shadow of a Doubt combines two genres beautifully. Initially, the "mystery" of the murderer is kept from the family members and the audience with subtle hints given along the way that lead to the identity of the killer. The second half becomes more "thriller" like with the tension that arises between the family members as the identity becomes known to some, while others are left completely in the dark.
All along, till the very end, Hitchcock still manages to leave a little doubt, both in the minds of the investigating niece and the audience as to the true identity of the killer. That Shadow of a Doubt that lingers in the mind till the final act is what makes Hitchcock one of my all time favorite film makers.