Here is a rather curious paper describing psychology researchers' attempts to investigate "spitefulness" - I think they define spitefulness roughly as "hurting others without any benefit to oneself". References the Stanford Prison Experiment. Concludes, more or less, that some people are spiteful, sometimes.
I have many reservations about the methodology used in this experiment (main one: not sure if the entire process really reflects any real-world motivations, and hence results might not mean much), but I thought it might be of interest to people on this site. Also, of the 30-odd references cited at the end of the paper some sound rather interesting and many are available online.