Why would we kill?

I recently watched a movie, a Hollywood telling of the story of the lost boys of Sudan.  It was sad, but at the same time, moving.  The people of that nation has long been in a civil war, with almost unimaginable killing.  One scene showed small children in hiding, while soldiers went through their village killing anyone they could find, men, women and children.  To me, this is incomprehensible.  On what reasonable grounds could there be justification for shooting anyone.
While the reality of man's fallen, sinful nature is ever before us, that reality is not justification for sin.  It explains it, but does not justify it.
The civil war in Sudan is, apparently, based on religious and tribal differences.  That is about all I know of those details.  There is also the reality of racism.  If someone is raised to believe that one of another race is subhuman, killing them is of no consequence.  If someone is raised to believe that one of another race is basically on the same level as vermin, then killing them is a service to mankind.  If someone has something that we want, such as land or resources, then killing them provides the means for taking it.
I hope it is obvious that all of these reasons are based on faulty worldviews.  All men are made in the image of God.  No one should be killed, executed or otherwise killed unless they are guilty of murder.
The answer is really very simple.  God calls us to love one another and treat others as we would be treated.  Everything else is secondary.