Humans are such imperfect creatures; even though they are the most intelligent of all living things on earth, they are quick to kill than to negotiate for peace. And this trend had continued for thousands of years... and this is true for all human races on this world.
When one cannot agree on something with another (e.g. gang, organisation, nations) ; eventually, there is no place for the "opposition" to remain alive in this world. At the end of the day, it is the victor of the war who justify the slaughter of lives... and that they were right to start this or that war in the first place. And even if the war was an unrighteous one, the dead can never argue or disagree again!