What meaning is Hitchcock trying to portray in 'Aborgast' killing scene Psycho film?

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.