Jaime, I strongly agree with you in the part that institutions have helped on reducing de violence, such as The United Nations was founded almost after the World War II, Now the countries respect such thing called diplomacy, at least in certain level.
On the other hand, I don’t think it is the only reason why worldwide the violence has been reducted. Another reason is the amount of impunity that existed before, and how, in many countries is more possible that if you do something violent, you will get caught.
My third argument on why violence has gone down, is that many countries have less access to arms that the access they had some centuries ago. Imagine you living in de Wild West, it was almost sure that you had one gun carrying almost everywhere, and you fought with some guy in a cantina, very likely this fight would end with someone dead.
Answering the main question of this topic, I don’t think that in our nature we are less violent, look at the videogames, that as you said (Pedro), people enjoy killing, also look at the masive murders in the USA (where they have access to guns), sometimes for simply pleasure. We might be as violent as we were before, but the circunstances have changed to be a more peaceful society.