The death penalty may do nothing to prevent real murder, but, as reader, I find its existence in the justice system provides some tension in a mystery.
If the book takes place in a country that does not have the death penalty, I certainly care just as much that the killer be brought to trial. What worries me a tad less is that some innocent person will be condemned for the crime. At least they have a fighting chance to prove their innocence. Prison isn’t a spa, but the innocent might live long enough to see the murderer caught.
If the death penalty exists, then I get very nervous about all those red herrings. If the wrong person is found guilty, the real killer probably goes snickering off into the sunset. No one cares. The one found guilty is executed. Justice assumed done. Or at least that is in my mind as a reader. This happens in real life so the fear has some basis in fact.
In the medieval era, justice was pretty swift and penalties rather more violent, even for minor crimes, than we usually have. Hanging was the common form of execution, and it didn’t produce a swift neck break of later eras. The victim died slowly from strangulation. The innocent? Many were executed. Half the Jewish male population of England at one time was hanged for money clipping under Edward I before he realized that much of the evidence against these men had been planted.
So, as a writer of medieval mysteries, I tend to keep this fear of the innocent dying as added tension in the books.
Do others find this a useful element in books? Looking forward to your thoughts!