I think that Hitchcock wants to bring out the truly horrific side of Norman Bates' schizophrenia. The killing of Arbogast is shown to be quite awful. The stabbing down the staircase, and the manner in which he is shown as he descends backwards helps to create an image of how awful Norman, as his mother, can truly behave. Additionally, the continued view of the knife over Arbogast's screams help to make the image feel really bad at what is happening. I think that this is what Hitchcock sought to bring out in his depiction of Arbogast's killing. It is also one of the first moments where we begin to see "Norman's mother" as a truly malevolent force. From this point on, the view of Norman and his mother become more shrouded under a cloud of suspicion and mistrust.