We can add other equally despicable acts such as torture, lying, slavery, terrorism, and environmental devastation. Although the act of war itself has not changed much philosophically in the past 3000 years or so, thanks to the evolution of military technology, the potential for destruction has increased dramatically. Today the United States and other technologically advanced countries have the technological capacity to completely destroy any major city on earth and render it uninhabitable for hundreds of years. Thanks to the advent of biological and chemical weapons, we also have the ability to kill every single inhabitant in any major city without destroying a single building. The task for philosophers and theologians has been to explore the intersection between war and morality and attempt to set limits to all this death and destruction.
Pacifism has many different forms.
Its leading premise is that war is morally wrong. Based on deontological principles they say that killing human beings is always wrong and that it reveals a stark disrespect for human life. Critics of ideal pacifism, however, are quick to point out that failing to defend oneself against an aggressor also reveals a fundamental disrespect for one's own life. Some pacifists are utilitarians and argue that war never yeilds a favorable balance between costs and benefits. If you look closely at any war it's really difficult to find clearcut winners.