...more than they uncivilize it?
First, an understanding: The justice system doesn't exist for the benefit of the general public, but for the benefit of those specific members of the public accused of crimes. We're very good at vigilantism, not necessarily good at vigilante -justice-, and from what I have personally seen, 90% of what police do is discourage vigilantism; I think the death penalty results in fewer vigilante killings than it involves in executions, particularly in states like Texas.
Understanding that the justice system doesn't exist to protect the unaccused innocent, but rather the accused innocent, a lot of what goes on makes a lot more sense. How many people have you encountered who "Just needed killing?" Whose sole claim to life is that a penalty exists for taking theirs?
I'm not talking about people who talk too loudly in a theater. People who engage in what I'll call petty violence; lawyers who file frivolous lawsuits, for example - who use the law both as a shield -and- a club, who let the thugs of government legally do what they themselves could not get away with. Or burglars in states without the right to protect your property.
Are the police making things better, there, by enforcing rule of law - or worse, by protecting people who turn rule of law into a weapon?