One particular reason why "The Case for the Defense" is a mystery is because we do not know who dies at the end.  This element fits this aspect of the mystery genre because the reader is unaware of who suffers death in the conclusion of the story.  Was justice served because the guilty brother was killed?  Was a further injustice perpetrated?  Is the brother who is weeping doing so out of loss or an irredeemable sense of guilt?  At the end of the story, these questions confound the reader.  It is in this regard where the ending of the story helps to constitute a mystery.  The reader is left with more questions upon the end of the story and there is a lack of clarity evident.  Its ending is a mysterious one.

Built from this is the entire question of who did the crime.  The court proceedings move into a realm of mystery when it becomes clear that there is a twin brother.  Just like the star witness, Mrs. Salmon, the spectators are unclear as to who committed the actual crime.  A transgression was committed, and yet it is uncertain who perpetrated it.  This is where another realm of mystery resides.  The certainty with which the legal system is literally thrown into a realm of doubt.  In such a condition, mystery is present.