Explain

The phallic symbol idea gives depth to the characters in the film. Often, Horrors seem to focus on the fear both being felt by the characters in the film, and the fear felt by the viewer but have little focus on the protagonist. In Psycho we are introduced to Marion after it is apparent she has had premarital sex, we make the assumption she is a very sexual person and when she is stabbed this emphasises and stays true to this assumption. While in Halloween the protagonist, Laurie is a very innocent and perhaps even nerdy character. It is shocking for the audience to find the same character who previously brought a pile of books home for the weekend is now the same character who is attempting to protect herself by stabbing another person. From this, we learn that Laurie is not as innocent as we first thought.
A knife as a weapon allows the director to create a very graphic violence and so is helpful in the slasher sub-genre. Other possible weapons such as a gun although will kill the victim they do not leave enough space for shocking viewing. In the opening scene to Scream Casey Becker (played by Drew Barrymore) is brutally murdered by ghost face telling her he wanted  "To see what your insides look like". This results in Casey being brutally cut open and hanged, showing her insides and setting the macabre atmosphere for the entire film.
 Not only does the knife as a weapon add to the physical brutality intended in a horror film, but it also has the underlying impression that anyone is capable of taking such an awful action towards another person. A knife is a very common household item, everyone has one and this adds to the shocking qualities shown in any horror film that uses this convention. It seems that Norman Bates (Psycho) did not think out his murders, as it was to an extent out of his hands, so the knife may have been his only option, a simple weapon. Although we are not given much information on Myers' reasoning in Halloween other then the opening sequence where we see a 6 year-old Myers murder his sister and a small back story from Dr Loomis, it seems to be an act of tradition. He used the knife as a child, and is continuing this sick act.