All human beings have both a good and a bad side. The good side often results in peace and unity, while the bad results in violence and destruction. Many times the bad side has been revealed but, the twentieth century has been the most violent and destructive era in history, creating a sense of pessimism throughout the world.
Many events have contributed to this growing feeling of lost hope for the world and shattered human faith in reason and progress. One major event is war, specifically WWI and WWII. War, whether one is involved in it or not, affects everybody. Families who have lost their husbands, sons, brothers, nephews, uncles, fathers, and even grandfathers in these wars surely must have felt robbed of the ones they loved. This feeling probably made them angry and lose faith in the world, especially the government. When someone loses faith, everything in the world goes wrong for them. They don't feel as if the world is fair, instead they feel as if the evil in the world outweighs the good. Now just imagine the feelings of the surviving soldiers who actually lived in the war. When most of them returned to their homes, they felt lost and misplaced. They had nothing to do with themselves. What could they do? How can someone who has gone through life altering traumatic experiences be thrown back into the world and be expected to live a normal and happy life? Their personalities and views of the world have surely permenantly changed. Life couldn't be the same for them, not after having to witness a close friend be shot to death and another suffocate while coughing out his lungs because he didn't put on his gas mask fast enough. Then having to carry their bodies to a wagon where already a mass of bodies is piled up and watching the rats eat away at the carcasses. It's enough to drive anybody insane, infact many of them did go insane.
It is understandable how seeing death after horrific death can blunt and turn cold the emotions of any person and make the pessimistic.