I don't remember ever seeing Frenzy ( but have it waiting on my DVR). Your description of it just sounds like more Hitchcockian droll black humor. At that point in his career he didn't need to conform to someone else's standards such as Selznick's quirks, the Hayes office or the need to make money. Perhaps he was just yanking people's chains with grotesque humor. It could be a kind of "in your face" to film critics who had dumped on him earlier in his career for far more tame material that they found objectionable. And anyone who panned a film for being boring, would have to eat those words for Frenzy.
My personal vote for worst (among major Hollywood films) goes to "The Paradine Case". I almost deleted it about 6 times before the long-overdue, "not with a bang, but with a whimper" ending. I can understand why he named the victim in Rope as "David".