Sydney Carton, a bitter alcoholic, is a complex, deeply anguished character. Critics have debated whether his last act is selfish or selfless.
Admittedly, on the surface, it is nothing but heroic and selfless to take the place of another person who is about to be executed and to do it to help the woman you love. Carton makes the ultimate sacrifice to save others. Many have thus seen him as a Christ figure, which is an idea Dickens reinforces in the novel.
At the same time, Carton spends much of his life feeling worthless. Although brilliant, he believes he has wasted his life. His inner torments cause him to be self-absorbed, creating a vicious cycle, as his dwelling on his misery only makes it worse. He wanders the streets at night (much as Dickens did) and states,
I am like one who died young.
If he already feels that he has died, some have argued, then it was perhaps not such a great sacrifice for him to take Darnay's place. It could simply be that he uses this opportunity to save Darnay as a way to selfishly escape from the responsibility of life while looking heroic. This is reinforced by the quote. While famous and stirring, the focus of it is on himself and what he is doing. The four "I" statements pile up. On the other hand, it is hard not to be deeply moved by Carton's deed.