One of the accepted conventions of mystery novels is that readers like them because the protagonist -- whether police detective, private investigator, or amateur sleuth -- is presented with the chaos of an unsolved crime out of which -- in the course of the novel -- order is restored by nabbing the culprit.
Conventional wisdom say that the writer of mysteries must make sure that Good conquers Evil and that all loose ends are neatly tied up. Some exception is made for ongoing series -- the main plot line may be resolved but there can be some questions left for the next installment.
Kind of like those weeds -- hoe them down today but give it a week and there they are again. The battle to bring order out of chaos is never-ending.