Hawthorne illustrates complex themes in The Marble Faun. They are themes of strong interest to Hawthorne. One is crime and punishment, related to Miriam convincing Donatello to commit murder. Another is innocence and animalistic nature. This is related to the third theme of humankind's fall from sinlessness into sin and mortality. The fourth is Hawthorne's idea of the "fortunate fall." He believed that mankind's humanity is inextricably linked to the ability to sin because it is only the ability to sin that makes us candidates to receive the Christian gift of God's salvation. (Remember that Hawthorne was a Christian so thought and wrote from a Christian perspective.)
One comment on the first theme (crime and punishment) is that, in Hawthorne's eyes, the pain and guilt that Donatello suffered after committing murder is the price of his admission to true humanity. This connects to the idea in the fourth theme ("fortunate fall") in that now Donatello is truly human and therefore a candidate to receive God's greatest gift to humankind--salvation from sin.
Hawthorne doesn't see a contradiction here (one wouldn’t need salvation had there been no Fall from sinlessness) because of the second theme (innocence and animalistic nature). Hawthorne paints the prefall era of innocent nature with which Donatello is bestowed as one that is animal-like. For instance, Donatello compares himself to a wolf:
I shall be like a wolf of the Apennines ...." Hawthorne also suggests this nature is one that is instinctual, without real rational thought motivating it: "[He] came close to Miriam's side, gazing at her with an appealing air, as if to solicit forgiveness. His mute, helpless gesture of entreaty had something pathetic in it, ... what you may see in the aspect of a hound when he thinks himself in fault or disgrace.
All of these are bound up in the fourth general theme of humanity's fall from sinlessness into sin and mortality. Hawthorne makes it clear in The Marble Faun that he imagines that sinless state to be subhuman; to be something akin to what we see in domesticated and pampered animals. For Hawthorne, the loss of such a sinless state was no loss at all. For Hawthorne, gaining the rational, reasoning, erring human mind and motivation was a gain well worth the cost. He imagines that sinless state to be as simplistic and as animalistic as Donatello: "He is not supernatural, but just on the verge of nature, and yet within it." Donatello is frozen in inconsequential beauty just as the Faun of Praxiteles is frozen in a similar state in marble.