I recently finished Harvard Professor Steven Pinker's most recent book called "The better angels of our human nature: Why violence has declined". This 1000 page book Pinker argues convincingly that all categories of violence (war, murder, assaults, and rape) has decreased substantially throughout history. What about the 2WW with 50 million plus deaths. While this was indeed the most destructive war in terms of the absolute number of dead people, it is not the most destructive if you look at the percentage. Genghis Khan for instance, in his raids reached a 40 million figure almost a thousand years ago, a huge proportion of the people around at that time. In other words, for the average individual it was way more dangerous in Genghis Khans time than when Hitler was alive.
Importantly the number of wars have decreased throughout history and since the 2WW there have been fewer wars than ever before (despite serious tensions during the cold war I may add). It is of course easy to think that there are a lot of wars because almost every war is covered in the media in a way that is quite new.
Likewise the murder rate has gone from 50-100 persons per 100.000 per year in the middle ages to an average that is lower than 10 persons per 100.000 per year in the west today (in Sweden where I live the rate is more like 1 person in 100.000 per year).
Why has violence decreased so much? Pinker argues, and after reading his book I agree, that the main historical factor is the development of states i.e. an uninterested third party that can settle disputes that would previously spark violence. To call a state "uninterested" may be an overstatement since in order to flourish it is in the states interest that its subjects do not fight. Apart from this rationale Pinker also presents a large amount of data showing that the establishment of a state is correlated with a strong decrease in violence.
The other main factor Pinker argues is responsible for the decline of violence is rational-abstract thinking. Today we live in a global world, and like never before we get to peek into other peoples lives, even if they live on the other side of the globe. It is almost impossible and it is definitely not socially acceptable to dismiss other peoples suffering just because they live in another country.
If this sounds interesting, listen to the TED lecture with Pinker where he gives you a short outline.
Another authors who has similar ideas although his explanation is somewhat different is Matt Ridley, listen to him below.
Pinker, S. (2011). The Better Angels of Our Nature. Viking Adult.