What is the moral, theme, or message of A Tale of Two Cities?

From the very beginning of the novel, Charles Dickens points out that we do not live in a world of moral absolutes: the best exists concomitantly with the worst, especially in wartime. He reminds us that political perspectives will always be tempered by personal convictions.
Among the French and the English (as among royalists and revolutionaries), there are both villains and heroes in the tale. Dickens does not argue the superiority of a given political system but rather aims to show that any system can be corrupted. Related to this idea is that people sometimes make decisions that are (or at least appear to be) not in their best interest—instead corresponding with their deepest principles. For example, Sydney Carton, who could have evaded the guillotine, gives his life for love.