There is never an easy answer to the question and it may seem a bit flippant to say the reason is that the world is enslaved to "sin and death". We, humanity, are so desirous (covetous) of other's that we resort to anything that will get what we want. Sometimes what we want is not an actual thing, but may be something like recognition, revenge, or the defense of what we believe. Therefore, to get what we covet (#10 in the Ten Commandments), we lie (#9), steal (#8), have affairs (#7), and, finally, we murder (#6). What generally holds community together is mutual respect and the following of the laws of that define how we should act with each other. When there is disunity and discord within the the community the foundations of that community seems to be crumbling.
When community begins to fight within itself about how the community is to be governed (like the Democrats and Republicans today?) there is usually a scapegoat who is singled out as the originator of the problem and that scapegoat is either expelled or sacrificed. Blame is placed on the victim for the violence within the community and when the problem is dealt with (expelled or killed) peace returns to the community. Perhaps (and this is a big perhaps) that is what the young man who did the killing in Tucson unconsciously felt: he thought that his world was coming undone, someone had to be blamed and then that someone had to be expelled from the community.
Unfortunately, in our modern world, the killing of a scapegoat does not bring peace to the community anymore. Yet, because we are enslaved to sin and death, we continue again and again to seek peace by violent means, both personally and nationally. However, violence only brings more violence and violence in the name of "Good" is no better then the violence of the evil and wicked.
What is the answer? Jesus Christ. In his life Jesus taught us how to love all people and how to love God. The Lamb of God (who takes away the sin of the world, John 1:29), the Son of God, God-With-Us, in his arrest, trial, scourging, and crucifixion shows us the nature of our scapegoating process: the innocent victim must die to bring peace (John 11:50). And even as Jesus hung on the cross he forgave his killers, his disciples, and us because we don't know what we do (Luke 23:34). But for those who have eyes to see and ears to hear, we now do know what we do and that process has lost its power. To those who believe in Jesus Christ he has given us something new to desire: God's love, acceptance, and forgiveness. With the power of the Holy Spirit we no longer desire what others have but we desire to do God will as a member of God's family. Why should we desire only God's way? Because God is love (1 John 4:7-12).
If, in his death and resurrection, Jesus has defeated sin and death why is there still violence in the world? Because sin, death, powers, and principalities haven't gotten the message yet. The power of sin and death to bring peace is waning and violence must be used more and more to try to hold onto its power. The outlook for the future seems bleak as violence seems to increase every year. Either the world and all of humanity will destroy itself in an extreme paroxysm or it will come to know the truth about God's love and seek the forgiving grace of God so that God's kingdom will truly come.
For more on humanity's violence and God's love see Paul Nuechterlein page on his core convictions: Nonviolence and the Christian Faith. It is long but well worth the read.
May God's will be done on earth as it is in heaven.