> Not counting military or police, how many people in the last
> 18 months saved their lives by killing or threatening to kill
> another person?
I'm not sure this is knowable with any amount of certainty. There are, however, attempts to answer a similar question: how often are guns used in self-defense during a crime? This Wikipedia page has a fairly balanced answer:
http://en.wikipedia.org/wiki/Gun_violence_in_the_United_States#Self-protection
A guy whose methodology has been questioned says 2.5M/year; another guy whose methodology hasn't been questioned much (and published in the American Journal of Public Health[*]) says about 64k/yr between the years 1987-1990, or in about 2 out of every 1000 crimes. I think it's safe to say that the real answer is "a lot more than you thought it was when you asked this question" ...
/jordan
[*] Abstract of http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1615397/?tool=pmcentrez It is well known that many crimes in the United States are committed with firearms. Less adequately documented is the frequency with which victims use guns in self-defense. We used National Crime Victimization Survey data to examine incidents where victims employed guns against offenders. Between 1987 and 1990 there were an estimated 258,460 incidents of firearm defense, an annual mean of 64,615. Victims used firearms in 0.18% of all crimes recorded by the survey and in 0.83% of violent offenses. Firearm self-defense is rare compared with gun crimes.