The number of people who died in the last century and the first part of this century by Wars that were the aftermath of Colonialism and due to Ideological struggle between Fascism, Communism, and Capitalism, and the number which will probably die in this century still to come. All of human history is fought with violence. But modern violence is on such a massive scale that it dwarfs other conflicts in history.
Another really sad thing is that we are destroying our planet, and other species at such an alarming rate, far faster than human destruction of nature has ever been able to go previously.
Another sad thing is that now that capitalism is unleashed on the planet via globalization in an unconstrained fashion the destruction, death only are accelerating.
I track all this back to the relation between Being and Existence in the Western worldview, which is unique in our worldview, and underpins the development of technology, which is underpinned by the nihilism in the worldview, which is unique to this particular worldview, that has become dominant and has destroyed countless others though colonization and world conquest.
Ours is a beautiful planet, and it is very unfortunate that no only will our kids probably be less well off because of the debt we have heaped on their shoulders, but also they will have a worse environment, and have to deal with the fall out of our hubris, neglect, destructiveness, over expansion of the economy, globalization of the world, and generally increasing world wide conflict.
The only way out of this dilemma is through the radical transformation of our society for the better, and that looks very unlikely. To have had such excess and wealth and to have squandered it and the opportunities we had is extremely unfortunate.