One person killing 10 people in one incident? And 10 people killing ten other people in separate incidents? The difference between them is that the mass killing is catalyzed in many cases by the same variable, a firearm, which can be regulated and thwarted (not in every case, nothing is perfect). Those 10 murder victims while it is tragic that lives were lost there is no variable that exists in every case that could help in thwarting future incidents. Now of course we could argue that the firearm is not the variable, but if we look at the frequency of events in other states with strict gun regulation we see that these massacre's are actually combatted pretty well by the legislation.
That's an awfully random reading of the evidence. How do we know that they were thwarted? Just because they don't happen as often (that we know about)?
I'm not sure what it is we are discussing anymore if I'm being completely honest. It may be because I've been sitting at the courthouse since very early this morning waiting for them to decide if I will serve on a jury which I don't think they are. I think the number of times a mass killing occurs is a good metric to work off of, is it perfect not really, but it's something.
Jury duty is a pain...for some reason ever since I became a lawyer I've never been called :) I guess to me it's just kind of arbitrary. It's like saying we should base policy on the number of killings that happen on Tuesdays.