Wouldn’t this just be the inevitable result of the growth of the human population? In Guns, Germs and Steel, Jared Diamond speaks about primitive societies of hunter gatherers being quite violent and between which many murders occur, for often spurious reasons. If you look at the progress of the human species for the relatively brief period of our existence for which we have any record you can see how this might change.
Since we are only somewhere between 50,000 and 100,000 years old as a species, it’s not likely, to my mind, that we’re much different genetically now than we were at the start, so the social factors must be the answer. When you have societies that are basically formed of a small (20-50 people) cluster of blood-relatives living off the land and roaming to wherever the food is, their only concern is going to be for each other. Any other clan is going to be competition for resources, so they almost inevitably become the enemy. As the population expands, and it becomes more difficult to roam, areas that can be settled and worked agriculturally become prime real estate and these new tribes, still composed of related individuals but far less directly (now up in the low hundreds of members or maybe a bit more) gradually end up competing for space. As agriculture is adopted, it becomes important to secure more land so that more fields can be worked to support a greater population. The clash of competition is the end result.
Cruelty works into this because the natural instinct to protect the family is just not there for other humans. When you are talking about a group of related people, they are far more likely to care about each other than strangers, and this attitude persists to this day. So as these tribes grow and clash, the weaker ones inevitably get absorbed by the stronger and these tribes grow beyond a related family. In order to live cohesively, ideas must take hold, to give the people some form of commonality. Religion and statehood have historically filled that void, along with racial identification. Having these things in common let these completely unrelated individuals find some common ground to meet upon and live together peacefully. From earliest Sumeria to ancient Rome the people were united by faith and/or citizenship. Yet still the foreigners outside the borders were fair game. Because of this it was easy to devalue those lives you held less dear (like the criminals that were forced to fight in the gladiator pits).
What I think has really changed things for today is the further development of long-distance travel, from Columbus’s day, all the way up to the advent of the internet of today. Insular nationalism of the past took a blow when the Portuguese first round the southern tip of Africa and were truly able to settle in the East and learn more of its peoples. Further voyages of discovery led to a different understanding of the make up of the world, and the incurable advancement of scientific thought made the discovery of our place in the solar system and universe seem even more humbling. The printing press brought knowledge to millions who had never had the chance before, and the discovery of electricity and telecommunications made communication exponentially easier than ever.
All of this, I think, gradually led to an understanding of the need, as the world became smaller and more integrated, to be able to work together. Societies that wipe out others are less likely to be trusted for things like trade and alliances. Cooperation and respect engender the same in return (generally) and are better tools for advancement than conquest and cruelty. I really think this is why we see far less outright cruelty among the general populace of the world today, because of this understanding of our need commonalities. Ideas and philosophies than really became prominent during the Renaissance, the advancement of science, and (at least in my personal view) the loss of influence of religion have all contributed to a better sense of global community.
Just my 2 (or maybe 3 or 4) cents.