It's just as well anyone watching this seems to have no knowledge of the source material. It is "based on" a novel by Nicholas Blake, in his Nigel Strangeways series. Blake is the nom-de-plume of the former poet laureate, C. Day-Lewis (yes, indeed, father of DDL). In the 15 or so novels, Nigel is a donnish sort, nephew of a big cheese at Scotland Yard, and occasionally "helps." A gentleman sleuth, in fact.
The Beast Must Die is one of the best novels, and was once filmed (respectfully) by Claude Chabrol. It involves the search by a man for the car and driver that killed his little boy in a hit and run.
This series makes the father a mother, and black, thus ticking two boxes. It makes Nigel a young cop with PTSD (tick). His turns with irrational tantrumising may have their place in some film about PTSD but add nothing except tedium to this story.
I keep watching to see what other horrors the series can inflict. I have long wished that someone would make a series for TV of Nigel Strangeways, but now live in terror that they will, featuring this preposterous and totally unrelated character.
Does nobody read any more? Is nobody capable of watching a period drama series any more? Poirot was a great success, and still holds up brilliantly. Poirot could quote Shelley and refer to a factory as the "fons et origo" of a man's success without being afraid an audience would not get it. That was as recently as the late 80s to about 2000. Morse could refer to opera. But it seems to be de rigeur in today's drama that everything should be up to date and "woke," and by no means referential.
I have rarely been so angry at any adaptation. And I am not Luddite -- I loved Baz Luhrman's Romeo and Juliet, and find Benedict Cumberbatch's Sherlock to be of as high a quality as the definitive Jeremy Brett series. But this Beast Must Die has just shut the door on what could have been a brilliant series of TV dramas with compelling stories and fabulous characters. I commend those who liked this series to the book, and any other Nigel Srangeways novels, all of which are available to read free on Internet Archive.