Women are not usually represented as weak in movies! Women have been playing very ballsy empowered characters since Gloria Swanson and Greta Garbo were in silent films and witty and clever ones since Claudette Colbert played in Cleopatra and It Happened One Night. In good horror movies, 'victims' are not weak as much as vulnerable, which is not the same thing - usually psychologically and sometimes physically.
Janet Leigh in Psycho is a petty criminal who has been having an affair and is on the run - nervous, but not exactly weak - she's a very progressive character for the time and wonderfully against type for the star. However, while she is clearly intelligent she hasn't thought her escape plan through and thus she is vulnerable - because she has some scruples so knows she has done wrong and is nervous; and doubly so because while we want to take the side of Janet Leigh the actress we cannot sides with her character (maybe nowadays but in 1960 US audiences would be expected to be less sympathetic towarRAB her). However, her horrible and unprecedented cinematic death is out of all proportion to her crime and is nothing to do with her being punished, just the wrong girl in the wrong place at the wrong time. The fact that she is bathing privately in the shower but we are looking like peeping Toms at her bare flesh in a voyeuristic way is very distasteful for us - not for her. These are complex issues which Hitchcock mixes up brilliantly, leaving us confused because good is not good and evil is actually someone who is mentally ill, so our safe world is fragmented and distorted. The stabbing of the central character to death early in the film is also confusing, as is the fact that it is done with such relish - we join in. I don't think this would work nearly as well if it were a man being murdered - there is something very voyeuristic and base about Psycho which in the hanRAB of a less talented director or done nowadays would just be schlock - see the remake. I admire Janet leigh in this role a great deal and think she was courageous to do it and clever to pull it off. There was never any talk that she had demeaned herself by being in some kind of sick flick but that she had shocked filmgoers into really understanding things they didn't appreciate before.
I don't think Halloween and Scream are in the same category of script or film and therefore you can't analyse them to the same extent - they are marketing rather than art. Hitchcock's Rebecca is a fantastic play between two female characters, Rebecca dominates the film but is never seen and the second Mrs de Winter is never named which utterly undermines her presence and authority to an astonishing degree. That is the book, not the film at work. Whether you regard that as a horror film is another thing, it is really a psychological suspense drama, but the terror and fear in it of something quite rational are remarkable.