A lot of the reasons the major countries became less barbaric, starting in the 19th and especially the 20th century had to do with Jewish influences (sometimes directly, or by way of goyim “appropriating” ideas from us), as well as the growth of mass media putting a spotlight on barbarity and making the barbarians look bad. There were always people who though that genocide was morally wrong and should be banned, or that drawing and quartering was savage – but it is only recently that these views can to predominate.
To Yabia Omer: You need to consider what was done to criminals until the invention of modern prisons in the early 19th century. If you think killing a bunch of children is barbaric, what do you think of American and British “ethnic cleansing” in the 17th to 19th century, where the murderers were hailed as national heroes. Baruch ha-Shem we live in a world in which criminals no longer plea bargain for a merciful death, and that genocide and torture result in one being labelled “Hostis humani generis” (“enemy of all mankind” – a legal term the until recently was only applied to some pirates and that only some of the time).