The belief that violence has increased suggests that the world we made has contaminated us, perhaps irretrievably. The belief that it has xxi decreased suggests that we started off nasty and that the artifices of civilization have moved us in a noble direction, one in which we can hope to continue.
The "XXI" in capitals mean 21 in Roman numerals, but I don't think the lower-case letters "xxi" here also refer to 21. It seems that the sentence is well understood if the "xxi" is removed. What does the very word mean here? Thank you.