What is the meaning of the short story "Markheim"?

Whenever we consider the "meaning" or "message" of a given
story we must remember that such concepts are extremely relative, and every work of
literature has a potential multiplicity of meanings that we need to be aware of.
However, when we think of this masterful example of the Gothic genre, it is clear that
the meaning revolves around the concept of the "double" or the "doppelgänger" that is
introduced. Consider how, once he has committed the crime, Markheim comes face to face
with a curious and fascinating figure, described as
follows:

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Perhaps there was a film upon his sight, but the
outlines of the new comer seemed to change and waver like those of the idols in the
wavering candle-light of the shop; and at times he thought he knew him; and at times he
thought he bore a likeness to himself; and always, like a lump of living terror, there
lay in his bosom the conviction that this thing was not of the earth and not of
God.

Note the insubstantial
way this "double" figure is described, and the way that Markheim feels at times that it
has a resemblance to himself and that he knows him. Markheim goes on to identify this
figure as the devil, and, to support his conclusion, this strange personage does seem to
tempt Markheim on into ever-greater depths of sin and persuade him to live his life
committing evil acts, however, this does not explain the rather curious change in the
expression of this guest at the end of the tale when Markheim determines to be master of
his own destiny and turn himself in. Consider how he
transforms:

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The features of the visitor began to undergo a
wonderful and lovely change: they brightened and softened with a tender triumph, and,
even as they brightened, faded and
dissolved.

Such a change,
linked with Markheim's determination to "cease from action" and thus escape his fate of
condemning himself to evil, indicates that this figure is actually a projection of
Markheim's repressed conscience, a creation of his own mind, that explores
psychologically Markheim's character and is able to "tempt" Markheim back to
good.

Thus this story is a powerful psychological
examination of the conscience of a criminal and how Markheim comes to choose to turn
himself in, thus laying down his evil life and stopping his slide into even more hellish
acts.