Modern society would say he has a martyr complex. The man pines away after a woman he knows he will never have and then sacrifices his life for that woman's beloved. Sure, in the spirit of the Romantic era his deed is seen as noble and virtuous. He's a veritable Galahad.

In the ironic post-modern world, however (tip of the hat to Paul Fussell on the ironic bit), we should be far more careful in assessing his character. What, exactly, separates him from those raving mad Christians who went out of their way to get themselves thrown to the lions, reveling in their self-destruction? At the end of the day, is his act really all that different from just putting a pistol to his temple and blowing his brains out on the rug? The side effect of saving Darnay doesn't affect the direct self-destructiveness. Was he really doing it to save Darnay, or was it a way to serve his own martyr complex? Is it not a valuable question?

If you don't like this, though, I'd be happy to discuss the spontaneous human combustion of Mr. Krook in Bleak House. The lurid nature of the whole affair is quite attractive to me.