We have entered an age of genocide and ethnic cleansing, as a persual of the news will make clear. If you are on the Left, you are perhaps most concerned about Israel. Juan Cole wonders if "Zionists" are talking about a "Final Solution" to the long problems with Palestine. He links to an Israeli writer who seems to be wondering just when genocide is permitted as a defensive measure, a question the answer to which will be disappointing to the author.*
If you are on the Right, you are probably thinking about ISIS/ISIL, who are waging a genocidal campaign against both Christians and Yazidis. This is being done according to the old forms: systematically killing the men and male children, taking the women and girls as sexual slaves. The Iliad was built around a war of this kind, as are some of the Old Testament books (especially the Book of Joshua). The Beowulf ends assuming that this fate awaits the Geats with their mighty king dead and their warriors humiliated. The Koran appears to advocate this in some of its verses as well. This was the way things were, and now it is the way that they are again.
The question is therefore not why the age of genocide has returned, but why it ever stopped. The answer is that there was a brief window of history in which Western Christian states were the chief world powers, and those states had developed a religious and philosophical tradition of restraining war's violence against noncombatants.
They wrote the rules of this tradition into the laws that are supposed to govern violence in our world, and they did something to enforce them or live by them. They failed at times -- especially the Germans did during the Nazi period, but there were other failures including our own treatment of certain American Indians -- but as a civilization they believed in these rules as if they were laws, and were willing to pay a cost to make them real. They believed in it so much that they wrote their laws as if "civilization" itself was synonymous with this tradition's understanding, though no such tradition existed in much older civilizations elsewhere. They used their power to compel, urge, bribe, or coerce the most of the world into signing documents attesting to these principles they called laws. True belief in these principles, though, was not as widely shared as the signatures would suggest.
The reason that the age of genocide has returned is not that the West has ceased to believe, although much of it has, but that it has ceased to be strong, or willing to pay a cost. For decades the last true force of the civilization belonged to America, with a few brave warriors from allied nations though those nations as a whole increasingly voted against strength. When the American people chose the same path, the return of genocide was inevitable.