The gun stuff has been talked about ad nauseam, but the saddest thing is we can change every law in the word and it still won't be enough to stop a lone wolf intent on killing one person like this. You can't get every gun off the street, every mentally unstable person in a hospital, etc. More should be done, of course, but if one person gets it in their head that they want to kill one person, unless they let it slip to someone, or the possible victim is guarded well (politician, for example), it is hard to stop it. Which sucks. Heck, just look at John Lennon.
That all aside, terrible tragedy.