An interesting plot element in a detective story is having multiple murderers. This can really complicate the life of the detective because each murderer may have a truly unbreakable alibi for the murder he didn’t commit. While the detective (and everyone else) labors under the assumption that one person committed both murders, the only viable suspects will probably have no motive.
As I think I’ve mentioned before, one of the really interesting ways of doing this is to have the two murderers murder each other, though one with some sort of time-delay mechanism such that he’s already dead by the time it goes off and kills the other fellow. (Poison is an excellent murder weapon for this case.)
Another scenario for multiple murderers occurred to me: a primary murderer and an after-the-fact accomplice who kills the original murderer to hide his after-the-fact involvement in the first murder. For convenience, let’s name our murderers: John and Steve.
John murders someone, let’s say his wife, Alice. John didn’t plan it out, though, and needs help disposing of the body and erasing the evidence, so he goes to his friend, Steve. Steve reluctantly helps John because he doesn’t want to turn him away, but on the other hand really wishes that John had left him out of it.
The detective begins to investigate and starts coming up with clues that point to John, but also to John having an accomplice, at least after the fact. This alarms Steve, who never wanted to be involved, who got nothing out of the murder, and who doesn’t want to see his life go up in smoke because of John’s bad decisions. Steve begins to think of how to get out of this, and the one solution he comes up with is murdering John. Only John knows it was Steve who helped him, and Steve does have an excellent alibi for when Alice was murdered. If Steve can make it look like John was killed by the same person who killed Alice, he’ll be home free.
This would make for a good mystery, I think, because the detective would first have to disentangle the two murders as not done by the same person, then figure out what happened in the first murder, and from there figure out who the second murderer was. It gives a nice progression of realizations and reveals without everything coming at once, which is the key to a really good mystery novel. (Short stories do better with a single denouement.)