A common enough plot device in murder mysteries is the murder being taken to be an accident or a suicide. This often follows from the murderer trying to disguise the murder as an accident or suicide; any number of things may lead to it being taken that way. The ones that I particularly have in mind, though, are the ones where the detective is unsure of whether it was murder or not.
The problem with this is one of genre—what with the story being a murder mystery, something has gone wrong if the murder isn’t actually a murder. Granted, this does come up from time to time, but such stories are almost invariably disappointing. They can be partially redeemed by the accident or suicide being disguised as murder as part of an attempted murder (framing an innocent person for the not-a-crime, or at least not-their-crime in the case of suicide). However, even this is only tenuously in the genre of murder mystery. The worst, though, is where a series of accidents served to obscure some evidence, which then led the detective to think that this was the one clue which didn’t fit. I get why such things exist—if something is never an alternative, there is no suspense—but it’s deeply unsatisfying for the detective to unravel a problem which he created. It’s not good when the best case is equivalent to the detective having stayed home and taken a nap.
Anyway, whichever way this goes, there is a further problem of genre: unless the book is in the wrong genre, we the reader know that the accident wasn’t an accident. This reminds me of something that James Cameron said in a conversation with his co-writer of Terminator 2. The quirk of the movie is that this time the terminator from the first movie is sent back in time by the human faction to protect a kid from a more advanced terminator sent back in time by the machine faction. However, this is only revealed about 20 minutes in to the movie; they shoot all of the scenes with the original terminator in an ominous way, as if he’s the bad guy. But, as Cameron pointed out, the problem is that in the leadup to the movie Arnold Schwarzenegger was doing hundreds of promotional interviews that started with, “So, Arnold, this time you play a good guy”—that is, it’s not like the audience was actually going to be fooled. So what’s the point?
And that’s a good question. I’m not sure that such pretenses really have a point. They can be a plot device to put the detective in an antagonistic position with the police—especially when the detective is himself the police, leading to the chief of police saying “you have 24 hours to prove this was murder”. They can be a plot device to put the detective in an antagonistic position with the family or other witnesses and suspects. But it just doesn’t work for the detective to be unsure that the death really was murder, because from the point of view of the audience it’s not an open question. If the detective is trying to figure out whether the death was murder, the reader is in the position of having to wait for the author to get to the good part.
Something I can’t recall ever having seen, which I think would be very interesting, would be a murder mystery in which the first “murder” was an accident but the second “murder” was a genuine murder. With the murderer hiding in the shadow of the accident which people take to be murder, he would probably be putting himself out of the reckoning because he has a forged steel alibi for the first “murder”. Only once that death was discovered to be an accident would his breakable alibi for the second murder come under real suspicion.
Given the sheer number of murder mysteries already written, I’m sure that this has actually been done. What I think is more common is a variation on it, where one person commits a murder, then a different person commits a copycat murder, hoping to hide in the first murderer’s shadow. I know I’ve seen that, at least in an episode of Numb3rs. Stretching the concept slightly, you can also see it in the Brother Cadfael story One Corpse Too Many. This is definitely an interesting plot for a murder mystery. It makes things nicely complicated. So complicated that it really can only be done in a mystery novel, not a short story. (The episode of Numb3rs, being a TV show and therefore structurally a short story, managed this with a multiplicity of murders that just made a single villain impossible and further had the solving of the individual murders happen off-screen.)
That said, I think it would be an interesting variation to have a murderer stumble onto an accidental death that he had an alibi for and set it up to be a murder, then during that investigation—or better yet, I think, once it has concluded in frustration—to commit another murder with the same modus operandi and other clues pointing to it being the same murderer.