I had a realization some time ago...after reading a book out of a children's series, no less. Most people probably think they could never kill another human being unless it was self defense. True, some killers do so because of insanity...but many are not so different than you or I. I would say that a great majority, if not all, of us have the capacity to murder. But we haven't because we haven't met the right circumstances--we haven't reached our breaking points.
I was discussing this a bit in another forum (which unfortunately was not very active) when someone directed me to this Radiolab podcast. The entirety of the topic of the podcast is not murder, but they do address murder at times. I was particularly surprised by the results of a survey given by a professor David Buss that asked this question. They go into this a little bit from 5:00-8:20 into the podcast, but then they end up going into the Milgram experiments (which I feel are not directly related).
I wanted to read more about Buss's survey to see if there were more questions, etc., but I couldn't find anything about it in particular on Google or my university's library search tool.