Murders in the U.S. have decreased steadily since 2006, dropping from ~15k to ~12k. Firearms murders, which made up 67 percent of all murders in the U.S. in 2010 have followed this trend, decreasing by 14 percent.
California, a state with one of the most strict gun laws had the most gun murders in 2010. 69 percent of all murders that year, according to FBI data.
The point is not that gun control reduces the number of murders: it's that making it harder to get hold of a gun makes it harder to kill somebody.
Anybody who wants to kill somebody badly enough will do it with or without gun control: you'll still be able to get hold of a gun regardless. What gun control will do, however, is make it much more unlikely that somebody will go out and act on their murderous feelings.
To the man with a hammer, everything looks like a nail. To a man with a gun, it looks like a valid solution to far more problems than it actually is.
And yet here we are: a man just shot up a school full of kids.
A gun is an effective way to kill somebody - regardless of what happens after the fact. Most of the time in these kinds of massacres, the perpetrator doesn't care at all what happens after the fact - and I'm willing to bet that in a lot of gun murder cases, the perpetrator was not thinking about the consequences / ramifications at the time they pulled the trigger.