How does the story show why humanity is driven to violence?

There isn't any violence in the story itself. The story begins with several guests debating whether capital punishment is justified. The guests, intellectuals, come to the conclusion that it is a medieval and out-of-date practice that should be retired, a view that the story continues to support until its end. Indeed, one could consider the fact that the lawyer manages not only to survive his imprisonment, but also to educate himself through it, a thesis defending the value of imprisonment over the value of capital punishment.