We delegate the responsibility to declare and prosecute war to our legislative and executive branches, and we put guns into the hands of the army and the police. Governments, in other words, kill on our behalf. This arrangement is a form of social contract, which means that governments are basically contract killers. In our more decorous democracies, meanwhile, we perfume our conversations with words like "justice" and "national security" to mask the odor of death.
More troubling, perhaps, is the fact that leaders don't necessarily hijack the political process in order to use violence. Swayed by fear and nationalism, a democratic society can agree, albeit with significant minority dissent, to a rollback of democracy (such as the USA PATRIOT Act) or a full-scale military invasion (into Afghanistan, for instance).
We believe that we are safer when the government can use violence against those who (we are told) threaten us at home and abroad. But perhaps the price for that security is too high: people executed under the death penalty and innocents who die in the line of fire overseas.