"JA wanted to portray both of them, Sir Walter and Elizabeth, as silly, spoilt and self centred people rather than evil or witchy."

What's JA's conception of evil, do you think? I haven't really thought this through, but I bet you could make a case that, for her, such vices as selfishness, lack of consideration for others, and the snobbish pursuit of rank and wealth at the expense of true human connection are precisely what constitutes evil. Many of the really nasty characters in her fiction -- e.g., Sir Walter & Elizabeth, the Eltons, Mary Crawford, Mrs. Norris -- are guilty of these kinds of sins, rather than of the more sensational Mysteries-of-Udolpho-type crimes.