Nobody seems to mention the reason I think accounts for a good deal of the drop, which is simply the perception that you're going to get caught is much higher. Up until the mid 90's there wasn't even DNA analysis. The world was a much simpler place, no cameras everywhere, cell phones, internet, and a myriad of other ways to be tracked. The rise of technology in law enforcement coupled with the non stop blasting of crime shows depicting criminals getting caught (e.g. CSI) has greatly increased the perception that you will not get away with murder. Today most sane people at least view it as something extremely risky and hard to get away with. Even to the destitute gun crime is not a very attractive option these days, whereas 20+ years ago it wasn't nearly as big a deal. Of course there's a handful of other factors, but I think the perceived likelihood of being caught is probably the most significant.
My theory is that the advent of a popular internet itself deters people from violence. No science to back it up, of course. But people spend more time watching TV and using the internet; it leaves less time to be outside getting physically rowdy with others and creating these heated moments.
