Things aren’t always what they seem. Disreputable, lazy good-for-nothings turn out to be saviors. Righteous, justice-seeking people turn out to be bloodthirsty thugs. In other words, war tends to confound most people’s expectations. Once blood starts spilling in the streets, telling the difference between right and wrong becomes extremely difficult. When the world turns upside-down, how do you decide what to believe? More important, whom can you trust? A Tale of Two Cities explores the agonizing consequences of revolution, such as how "freedom" can too easily become another tagline for fanaticism. Sure, revolution can bring freedom – but at what cost?
Because Charles Darnay’s sense of morality is completely fully-formed (and never troubled) at the beginning of A Tale of Two Cities, he never becomes a truly interesting character.
In the turmoil of political upheaval, characters whose morals remain constant become the only ones on which we can rely. That’s why we like Charles Darnay so much.