One of the ways that Dickens develops the theme of humankinds capacity for violence is through the separation of the classes in both cities and the ways that it affects people's view of mankind. One of the most horrifying examples is that of the Marquis riding towards his massive chateau and when his carriage runs over and kills a child thinking nothing of it and tossing the man holding the dead child a coin as if to make up for the loss.
This is one specific example but throughout the book the reader finds examples of the way that the aristocracy can dismiss the cares and even the lives of the peasants or the working class people. The Marquis himself suggests that the poor are better off dead, demonstrating an utter lack of humanity and morality.
The capacity for violence and lack of feeling is perhaps most closely linked to the belief of the upper class that were was a natural or genetic superiority to them that elevated them above the status of the commoner.