There is no part of writing a story which is truly easy. That said, different sorts of stories have easier and harder endings. This comes from the nature of the story; in some stories the meat of the story is in the main part of the story—or, rather, the reader eats the meat during the main part of the story. Action stories are like this; the meat of the story is the action. When you come to the end, the reader is full and only needs a light desert to finish the meal. That is, one just needs a happy ending which fits.
Murder mysteries, by contrast, delay the meat of the story for the end. There is considerable variation in how murder mysteries are structured—not all of them collect clues in the beginning then gather the suspects together into the accusing parlor for the detective to explain the solution. Many mysteries—often my favorites—make deductions along the way. The mystery unfolds as the story progresses, though often with a final clue that solves the final piece of the puzzle in the end.
But even if the deductions are made throughout the story, they are provisional; what a clue means is rarely certain. All the more so because a clue can generally mean many things. That is, a single clue can be explained by many actions. That one has a plausible interpretation for a clue, considered in isolation, does not mean that it fits with the rest of the story.
It is possible to have various clues which require the murderer to be tall one moment and short the next, striking out in blind rage one moment and coldly calculating the next, trying to disguise the murder as a suicide one moment and trying to frame somebody else the next.
There is, therefore, the moment when all of the deductions, though made earlier, must be put together into one cohesive whole to see if the deductions are compatible with each other.
There’s a good example of this in the Lord Peter Wimsey story Have His Carcass, toward the end of the book. The story told by the clues assembled so far was that of a long-laid conspiracy which involved almost split second timing for the murderer to get from the moment one witness left him to ride hell-for-leather on a horse over the surf, leap upon a rock, slash a man’s throat, leap back upon the horse, and ride hell-for-leather back to his campground with only moments to spare before another witness he could never have predicted would see him. They had timed the actions and it would be, technically, possible. Any given supposition fit the facts immediately next to it, but when told from beginning to end, it simply made no sense. People don’t lay in intricate plots to have to madly dash about for no reason that they could have foreseen. (For those who haven’t read the story, this isn’t the end; there is a twist left to discover that reveals the real story, which does make sense. The detectives find it because they reject the story I just described as too implausible and keep searching.)
Every action is made up of complex parts and simple parts. A story—in this case, the story of the murder (that is revealed within the story of the detection)—can fail either by being complex where it should be simple, or by being simple where it should be complex. It is only by telling the story all the way through that we can see if it has the proper proportions.
Of course, one way this can be difficult in a murder mystery is for the solution to simply make no sense. This can be a problem for some authors, from what I’ve read, but simply can’t be for me because of the way I write murder mysteries. I start, not with the detective story, but with the story of the murder. I write that out as a simple prose story; the motivations, the plan (if there is one), what the murderer did write and what mistakes he made that left clues—all of this gets written out. (To give a sense of detail, so far it has tended to be between 5,000-10,000 words.) Since I start with the story of the murder as one that makes sense, when I finally re-tell it within the detective story it does hold together at least at the factual level and with regard to consistency of character motivations, skill level, height, strength, etc.
But though the way I write mysteries guarantees (as human affairs go) that the solution is free from plot holes, none the less it can still have imperfections as a story. A motive may be insufficient, a killer too cold blooded or not cold blooded enough, the risks taken might be too daring or too safe—my way guarantees that it is at least a coherent story, but it cannot guarantee that it is a good story. Nothing a human being can do in this world can guarantee that one writes a good story.
And here we come to why it is hard to write the endings to murder mysteries. One must, perforce, hold the story of the murder up for examination. But when any work of art is held up for examination, a gap between the perfect thing the author imagined far off and the real thing which he actually wrote becomes apparent. The gap can be bigger or smaller, but it cannot be entirely closed by a fallible human being. To finish a mystery is thus to face this gap, which is painful.
There is no cure for it, one must simply slog through it. As in all things, one must do one’s best and trust God. The only viable alternative is giving up.
(Though, as a note of explanation for many works of art, drugs and hubris can numb this pain enough for an artist to publish. They’re not as good as trusting God, of course, but in this limited respect they will get the job done. This explains why one sees so much of the one, the other, or both, on the part of artists.)