pity they killed off the original detective.
It was his decision. He wanted more time with his family, which he couldn't move to the Caribbean. I do like the new guy, but the chemistry isn't the same. However, DiP does have some of the best "howdunit" plot writing. Figuring out how the crime was committed is more in the Conan Doyle/Christie style.
It seems that defines the difference in darkness of these shows. In some, the crime, itself, is so gruesome that it dominates the plot. I.e., a la the Ripper, "Who would surgically disembowel several women." Then the search is for someone evil or psychotic, or for a super-villain. In other shows, the crime is ordinary --a simple stabbing, but while the victim is giving a speech in front of a thousand people and no one sees. Columbo flipped that on its head by showing the audience how it was done, but the fun was watching him figure it out.
The George Gently series is great. If you haven't already, check out the Foyle's War series. The WW2/Cold War context adds another dimension to the crimes and their motivations.
"A man is rich when he has time and freewill. How he chooses to invest both will determine the return on his investment."