How is morality explored in Moll Flanders?

In his preface to Moll Flanders, Defoe states that his intention is to convey a serious moral. Yet, explicit moralizing is notable by its absence in the relating of Moll's bawdy, picaresque adventures. Right throughout the story, Moll becomes caught up in all manner of escapades, none of which are in any way illustrative of the Christian moral life.
Moll is a serial thief and prostitute; worse still, from a moral point of view, she is an unrepentant serial thief and prostitute. Yet she does live by a moral code of sorts, often trying to rationalize her socially unacceptable behavior.
For instance, after she's robbed a drunken rich man she's just slept with, she convinces herself that perhaps the experience will prove instructive to the unfortunate gentleman. Perhaps, she reasons, he'll be less likely to cheat on his wife in future. Not surprisingly, that doesn't happen, but we can at least understand how Moll's mind is working. Essentially, she seems to believe that good can come from bad, moral actions from immoral ones.
Moll tests out this principle again when she steals from a young girl walking home by herself:

"I had given the Parents a just Reproof for their Negligence in leaving the poor little Lamb to come home by it self . . . ” (p.154)

In other words, it's not her fault that this little girl has been robbed; it's her parents' fault that they let her walk home all by herself, carrying such valuable objects. Maybe this unfortunate incident will teach them a salutary lesson.
Defoe was intelligent enough to know that treating Moll Flanders's story as an extended homily wouldn't have been anywhere near as interesting for the reader. Moll is his protagonist. As such, it's necessary for her to be made recognizably human. We cannot sympathize with Moll's criminal dishonesty, nor with her disingenuous self-justifications. Defoe doesn't expect us to.
However, we can at least understand her actions, even if we don't in any way accept them. In other words, we are induced to feel empathy, not sympathy. Defoe's method of telling the tale is all the more effective for being descriptive rather than prescriptive. It also provides us with a much more honest, more realistic portrayal of the criminal sub-culture in which Moll lives, moves, and has her being.