Loss of hope. If you think there is nothing beyond this life and this is all we get, there's no reason to be patient and wait for some after life justice and there's also no fear of doing something immoral. Rules and laws become arbitrary without some kind of moral foundation.
Also, if you think that there is no chance that humans will ever evolve beyond what we are today (and, let's be honest, there's very little evidence that people will ever evolve to be completely good), why not just burn everything down? Just fully embrace Darwinism. Survival of the fittest. Take out the useless or your enemies and at least make your own life a little better. Fuck society. Just look out for #1.
Sometimes I wish I could be like this. I think it would be easier to be angry than depressed over the state of the world. But, at heart I'm still an optimist and I still want to believe that humans can evolve to be good over time. I'm probably wrong, but I want to believe. Maybe others choose to be angry (or at least indifferent) rather than depressed.