Quote 4
He knew very well, that in his horror of the deed which had culminated the bad deeds and bad reputation of the old family house, in his resentful suspicions of his uncle, and in the aversion with which his conscience regarded the crumbling fabric that he was supposed to uphold, he had acted imperfectly. He knew very well, that in his love for Lucie, his renunciation of his social place, though by no means new to his own mind, had been hurried and incomplete. (2.24.58)
Darnay’s sudden decision to return to France is based upon a strong sense of responsibility for his family’s actions. To his mind, inaction can be as immoral as bad actions.
Quote 5
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. With this uneasiness half stifled, and half reproaching him, he had been brought to the pointed comparison of himself with the brave old gentleman in whom duty was so strong. (2.24.61)
The type of introspection that Darnay shows at this moment marks him as our "hero." He’s just so… good. It’s interesting to note that we don’t get a similar moment of reflection when Carton contemplates heroic action.