War and violence are leading causes of human fatalities in recent times (the 20th century). There can be many reasons for war and violence; for example hate crimes (against an ethnic group or country, etc.), religious fanaticism (can also be considered as a hate crime against followers of a different religion), colonial tendencies (nations fighting each other to expand their boundaries), control of resources (like oil, water, minerals, etc.), against women, etc.
Depending on the cause of war and/or violence, it can be justified. Think about people resorting to violence and war to protect themselves and their nation against an oppressor. Examples can be drawn from the freedom struggle of a number of countries around the world. If the cause is strong enough, say abolishing slavery or empowering women, war and violence may be resorted to.
Interestingly, the causes that may justify the war and violence are open to interpretation. For example, in some cultures, the role is women is so defined that it may appear as a lack of human rights to citizens of developed countries. As is said, "one man's terrorist is another man's freedom fighter."