According to this article by Stephen Pinker,

Violence has been in decline over long stretches of history, and today we are probably living in the most peaceful moment of our species' time on earth.

In the decade of Darfur and Iraq, and shortly after the century of Stalin, Hitler, and Mao, the claim that violence has been diminishing may seem somewhere between hallucinatory and obscene. Yet recent studies that seek to quantify the historical ebb and flow of violence point to exactly that conclusion.

Why?

The other major challenge posed by the decline of violence is how to explain it. A force that pushes in the same direction across many epochs, continents, and scales of social organization mocks our standard tools of causal explanation ... No one knows why our behavior has come under the control of the better angels of our nature, but there are four plausible suggestions.

The first is that Hobbes got it right. Life in a state of nature is nasty, brutish, and short, not because of a primal thirst for blood but because of the inescapable logic of anarchy ... These tragedies can be averted by a state with a monopoly on violence, because it can inflict disinterested penalties that eliminate the incentives for aggression, thereby defusing anxieties about preemptive attack and obviating the need to maintain a hair-trigger propensity for retaliation.

[A]nother possibility: that the critical variable in the indulgence of violence is an overarching sense that life is cheap. When pain and early death are everyday features of one's own life, one feels fewer compunctions about inflicting them on others. As technology and economic efficiency lengthen and improve our lives, we place a higher value on life in general.

A third theory, championed by Robert Wright, invokes the logic of non-zero-sum games: scenarios in which two agents can each come out ahead if they cooperate, such as trading goods, dividing up labor, or sharing the peace dividend that comes from laying down their arms, ... because other people become more valuable alive than dead.

[Fourth, e]volution [may have] bequeathed people a small kernel of empathy, which by default they apply only within a narrow circle of friends and relations. Over the millennia, people's moral circles have expanded to encompass larger and larger polities: the clan, the tribe, the nation, both sexes, other races, and even animals. The circle may have been pushed outward by expanding networks of reciprocity, ... but it might also be inflated by the inexorable logic of the golden rule: The more one knows and thinks about other living things, the harder it is to privilege one's own interests over theirs.

I suspect (3) is the major player, but who knows?