I’ve been thinking about the popularity of murder who-done-it in books and tv/movies for awhile. Interestingly, I didn’t reach the conclusion you did, about looking for shocks!
my (current) view is that it is an easily definable issue to watch/be involved in. When you read a book or watch tv/movie, you get engaged in a storyline. Almost all have a ‘what will happen’ element. In murder mysteries, the what will happen is a clearly defined ‘who did the crime?’. In chick lit the ‘what will happen’ is will they get together etc. In medical shows it is again like detecting ‘what is the illness and will it kill them before we find it’ (Interestingly tv show House was written with the premise of House = Sherlock and Wilson = Dr Watson, and it’s really solving medical mysteries).
Murder mysteries are essentially like solving a puzzle or a game - a clue here, and clue there, can you guess. You could do the same with other crimes, but with murder the victim can’t speak, adding to the mystery and making it longer, to figure out what they saw and their hidden backstory bringing a murder in. Eg if it was just an assault and not a murder, and you
were watching “Midsomer Assaults” you’d just ask - Freddie who could have give you the black eye? “Well, I actually saw them, it was Helen down the street who I am competing with over best orchid at the fair next Monday, as unknown to you I took up orchid breeding in the garden shed and have been joining orchid societies.” If the person was murdered, they aren’t there to tell, and would have to uncover all that as part of the mystery.
Plus solving the mystery isn’t depended on good behaviour or being attractive, so you can have blunt or rude or old lady (Marple or Vera), old man (Poirot), on the spectrum (Sherlock), average bloke (Barnaby), young female, middle aged female (Agatha Raison), upper class (books of Queen solving crime), ex-military (Strike) lower class etc detectives but they get the job done so everyone forgives them all faults. I think this appeals to people reading/watching as it’s a meritocracy they can see themselves there, as opposed to say chick-lit where the people are attractive and within a certain age bracket and money bracket.
Finally, they rarely show any violence and almost no emotional repercussions, upset family, businesses closed down, children left without parent etc, so you don’t have any bad emotional
feelings to think about. They also are usually in a bit of a morality tale, where the person may have done something to deserve it, so you don’t even have to feel bad about it.
That’s my current view anyway.