Miss Marple/WMG

Miss Marple is Nemesis, the ancient Greek goddess of Retribution.
This is joked about in the beginning of the story Nemesis, but what if it were the case? It would explain how she was always around when murders happen and how she has the powers to solve them.

And yes, there should be a Wonder Woman crossover.
 * She claims so herself in A Carribean Mystery

Miss Marple is a sort of Psychopomp who guides those facing death and tragedy.
She shows up at murders and helps everything work out. Maybe she's an angel of mercy and leads those who remain to resolution. Maybe she's an angel of death and drives people to murder because of her presence. Maybe she's some kind of demon or deity who just does it because it's fun. But come on, how do all of these murders manage to involve her all of the time? It has to be something!
 * Supporting evidence: One of Agatha Christie's other detective characters, Harley Quinn, actually is by Word of God a supernatural figure who shows up where he's needed. If one, why not another?

Miss Marple is a Serial Killer. Maybe.
Wherever she goes, people die, but she always manages to pin it on somebody else. The death rate around St. Mary's Mead is also a little high...

Obfuscating Stupidity is not a sign of niceness, you know. Especially in anything written by Agatha Christie.

Miss Marple is not an expy of Caroline Sheppard from The Murder of Roger Ackroyd--she is Caroline Sheppard.
The reason that Poirot allowed the killer to commit suicide rather than being arrested in Murder of Roger Ackroyd was out of respect for Caroline and fear that she would be disgraced by the revelation of the truth. So it seems highly likely that if the manuscript of that case were published, the names of all people and places would have to be changed first. According to this theory, "King's Abbot" was actually St. Mary Mead, and "Caroline Sheppard" was actually Miss Marple, with just enough details about her changed so that her neighbors wouldn't recognize her if they read the account of the crime.