In a later conversation with his partner Mr. Stryver, the narrator calls Carton a "jackal" because while Mr. Stryver very deftly presents each case and gains all the credit, it is Carton's legal acumen which helps them win, referencing how jackals help lions with kills while the lions take all the glory.
Several scenes make it clear that Carton is an alcoholic filled with cynicism and self-hatred due to what he sees as his wasted and empty life.
Lucie Manette and Charles Darnay eventually marry, increasing Carton's self-loathing all the more, as he had developed an unrequited love for her.
In an uncharacteristic fit of sincerity, he respectfully admits his feelings to Lucie, saying that though he considers himself unworthy of her affection, she has nevertheless inspired him to try to make something of his life.
However, this is the time of the Reign of Terror, and Darnay is arrested and eventually sentenced to death for being an aristocrat, although he had long before cut off relations with the Evrémonde family.