I have read suggested somewhere that Sidney Carton had syphilis. Dickens could not be explicit about sex matters. I suspect often when people hate themselves it has something to do with their sexual proclivities. Sydney Carton is heterosexual, so maybe his problem is that he cannot keep himself away from the prostitutes. If that was the case he may have contracted French disease. Or he may just despise himself for seeing prostitutes, particularly if he has had a religious upbringing. Sydney Carton is clever (he's another lawyer, isn't he), he has enough money, and if he looks like Charles Darnay then he's handsome too. Why doesn't he think he can get himself a nice, young lady like Lucie Manette?