I have long been a big fan of mystery stories in the British style, starting as a young age by reading all the Sherlock Holmes stories and as many books as I could get my hands on by Agatha Christie, Ngaio Marsh, and Dorothy L. Sayers. I have continued this into my adult years, though less so with books and more with TV shows and films.
One of the staples of the genre is the surprise ending, where the killer (there is usually at least one murder) is revealed to be someone wholly unexpected. In order to create the surprise, there have to be red herrings along the way that point to other people as possible suspects with motives and opportunity. The best stories are those where the ending is plausible and the various loose ends are tied up. This is not easy to do which is why writing good mystery crime fiction is hard.
What I have noticed recently is that writers sacrifice plausibility in their attempt to keep the big reveal until the very end. They also tend to pile on the murders. In my opinion, any story that has more than two murders and piles on their gruesome elements is likely to due to lack of imagination by the writer and an attempt to create excitement when other means fail. I recently watched the much praised British police drama Marcella and the plot holes were simply outrageous. The revelation of the murderer was a big surprise no doubt but it was also totally unbelievable.
There are three necessary conditions that crime investigators look for: motive, opportunity, and means. The writers of Marcella sacrificed opportunity, and the motive and means were also shaky. Also, there were a huge number of coincidences, inexplicable events, and lack of consistency in character behavior, always a sign of poor plot construction. Furthermore, there was no likable central character to hold things together. Pretty much every major character was either unpleasant or sleazy or psychotic. It reminded me of the recent series Sherlock in going over-the-top and becoming almost a self-parody in its attempts to shock and amaze.
In contrast, season four of the show Shetland was far superior. It was tightly constructed and the characters were well drawn and consistent. The ending was a surprise but not far-fetched as it was in Marcella. And there was no gratuitous violence, with the murders taking place off-screen and the bodies not even shown. Good drama does not need those prurient elements.
One of the things that the Sherlock Holmes stories showed was that the fascination mostly lies in how the detectives figure out the case, not in the surprise. This was a feature of the Columbo series too where you knew the murderer right from the start. In the Holmes stories, you did not need a murder or even any crime at all to find the story gripping. What is really necessary are central characters who are well-drawn and act consistently. The few episodes that I saw of the other Sherlock Holmes TV series Elementary was pretty good but unfortunately they are not streamed on Netflix.