motivated by compassion.

However, was it necessarily wise to go to Paris at that time? No, because Darnay's position as an aristocrat pretty much ensured that he would be arrested the moment he set foot in France, so any attempt to save Gabelle was doomed. Even Defarge wonders why Darnay would be so foolish as to return to revolutionary France.

So, Darnay's decision to return says much about him as a character: it reveals his noble spirit and compassion, as well as his naivete. It was ultimately an unwise decision, even if it was motivated by good intentions.

In Chapter X of Book the Third of A Tale of Two Cities, the reader learns that Dr. Manette was visited by the wife of one of the Evremonde twins, "a good, compassionate lady," who hoped to avert

"the wrath of Heaven from a House that had long been hateful to the suffering many."

Concerned, too, that if no atonement be made, it would one day fall upon her innocent son to do so; therefore, the Marquise Evremonde  declared that she would do all that she could to make amends.

Now, fatefully, the "loadstone" beckons Charles Darnay, ne Evremonde, back to France.  Having disavowed any connection to the cruelties of his family, Darnay, nevertheless, has tried to make amends by no longer taxing the peasants. And, he feels obligated to defend his former tax collector, Gabelle, who has been arrested by the bonnets rouges of the Revolution, accused of acting for an emigrant. So, Charles Darnay resolves to return to France in order to vindicate Gabelle. Another motive is Darnay's idealistic thinking that he can accomplish something positive by influencing the radicals to more reasonable actions:

Then, that glorious vision of doing good, which is so often the sanguine mirage of so many good minds, arose before him, and he even saw himself‚ in the illusion‚ with some influence to guide this raging Revolution that was running so fearfully wild.

Certainly, his sense of obligation emanates most strongly from the influence of his mother who seeks atonement for the Evremonde name. 

Charles Dickens symbolically entitles Chapter XXIV of Book the Second "Drawn to the Loadstone Rock"; the loadstone, a variety of magnetite possessing a magnetic polarity, symbolizes the pull of France to Darnay in his guilt over the sins of his father and, especially, his uncle. Added to this guilt, Darnay cannot allow Gabelle to suffer for the injustices of his family.

Like the mariner in the old story, the winds and streams had driven him within the influence of the Loadstone Rock, and it was drawing him to itself, and he must go.

His personal sacrifice of returning to France in response to the written pleas of Gabelle to show his expiation for the mistreatment of the peasants at the hands of the Evremonde family, of course, serves to further the narrative and provide Darnay's doppelganger, Sydney Carton, his opportunity for redemption, as well. But, Carton's main purpose is to "stay bloodshed," and to "assert the claims of mercy and humanity."

In Book II of the classic historical novel A Tale of Two Cities by Charles Dickens, Charles Darney inherits his family's noble title when his uncle, the Marquis St. Evremonde, is murdered in his bed after running over a peasant child with his carriage. Darney, however, is content to teach French language and literature in England.

Darney's decision to return to France is stirred by a pitiful letter he receives from Gabelle, a servant who has been helping to oversee the estate. Gabelle has been imprisoned unjustly, and his house has been destroyed. He has been accused of treason. He begs Darney to come and help him.

Darney is exceedingly upset by this injustice toward Gabelle, "whose only crime was fidelity to himself and his family." He realizes that when he inherited his title, he did not properly oversee "the renunciation of his social place." Under written orders, Gabelle had been holding the estate as best he could and generously giving what could be given to the people, but Darney feels that it is his duty to return to Paris and set things right.

Everything that arose before his mind drifted him on, faster and faster, more and more steadily, to the terrible attraction. His latent uneasiness had been, that bad aims were being worked out in his own unhappy land by bad instruments, and that he who could not fail to know that he was better than they, was not there, trying to do something to stay bloodshed, and assert the claims of mercy and humanity.

In other words, Darney feels compelled by his conscience to go to Paris to rescue his servant and do whatever he can to make the situation better there.

In The Tale of Two Cities, Charles Darnay is a native of France who is now happily living in London.  When he was last in France he denounced his claim to his aristocratic title and home. The Revolution had not yet begun, and he did what he felt was right to absolve himself of his home and heritage--everything but make it official, that is.  Things changed after he left.

Charles must go back. What he doesn't understand is how things have changed and the thirst for aristocratic blood in France since he was last there.  If he had known, he still would have gone; however, it's likely he would have gone about it differently.

In addition to the previous answer, Charles Darnay's going to England also represents his identifying more with the English culture, which is presented as less violent than the French, both pre- and post-revolution. In A Tale of Two Cities, the cities of London and Paris, and consequently the countries of England and France, are contrasted in their political/cultural identities.

Darnay denies his noble French heritage and the baggage that comes with it: he despises how aristocrats like the Marquis St. Evrémonde abuse those lower on the social totem pole than themselves.

By returning to England, Darnay is choosing to make a new life and identity for himself, not one built on the victims of the French upper classes. He marries Lucie Manette, whose father was a victim of Darnay's family, and in this way, is being reborn as another person, much like Sydney Carton will later be in giving up his life to save Darnay.

After catching up with his wicked uncle, the Marquis St. Evrémonde, Charles Darnay returns to England. On the face of it, this seems like quite an odd decision. For one thing, Charles came within an ace of being executed on a trumped-up charge of high treason. If it hadn't been for his quick-thinking lawyer and a fortuitous resemblance to Sydney Carton, he would've been toast.

But ever the romantic Frenchman, Charles has decided to remain in England primarily for reasons of the heart. He's fallen head over heels in love with Lucie Manette, another French expat, who lives in London with her father, a former prisoner of the Bastille. Charles may have had a close shave with the English criminal justice system, but at least he was acquitted, which would never happen to this young aristocrat under the tribunals of Revolutionary France. In choosing to make a life for himself in England, then, Charles isn't just listening to his heart; he's also using his head, a head that would be lost on the guillotine were he to remain in France.

Darnay makes the decision to return after receiving a letter from his former servant begging for help. Darnay used to be a marquis and owned a considerable tract of land that was looked after by Monsieur Gabelle. After Darnay renounced his title, he left Gabelle in the position of watching the Darnay land, which untimately resulted in Gabelle's arrest for siding with the aristocracy. Darnay feels that he should have tied up the loose ends of his actions in a way that wouldn't have left people vulnerable. Darnay is torn and does not want to return and put himself at risk, but he feels that it's necessary in order to clear his conscience. Gabelle's only crime was being loyal to Darnay even when Darnay himself was hardly being loyal to himself—relinquishing his title, running away, and leaving Gabelle to pick up the pieces. This shows that he has decency and dedication to his old servant.