But if you harm people regardless of whether they've harmed you first, it doesn't disincentive people from harming you. It only does if you're more likely to harm people who have harmed you than people who haven't.
But if you harm people regardless of whether they've harmed you first, it doesn't disincentive people from harming you.
Indeed, all else being equal it gives an incentive. Hurting you reduces your ability to do harm.
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0041812
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.