Cruelty, violence, badness... This episode of Radiolab, we wrestle with the dark side of human nature, and ask whether it's something we can ever really understand, or fully escape.

We begin with a chilling statistic: 91% of men, and 84% of women, have fantasized about killing someone. We take a look at one particular fantasy lurking behind these numbers, and wonder what this shadow world might tell us about ourselves and our neighbors. Then, we reconsider what Stanley Milgram's famous experiment really revealed about human nature (it's both better and worse than we thought). Next, we meet a man who scrambles our notions of good and evil: chemist Fritz Haber, who won a Nobel Prize in 1918...around the same time officials in the US were calling him a war criminal. And we end with the story of a man who chased one of the most prolific serial killers in US history, then got a chance to ask him the question that had haunted him for years: why?