I don't know why to be scared more then 1 month before? It can always happen, that war crimes or psycho crimes are commited. Everyone is not more or less safe then 1 Month before.
My guesses:
Some NRA (Weapon) guy is pissed about the new weapon laws (which was rejected today)
Some people are unhappy that Obama is still president (you know "black communist")
Some madmen just wanted to kill people
Someone is unhappy with the living conditions in America (tent cities after the crisis etc.)
Less Likely:
Some people are pissed about the torture in Guantanamo which increased lately
Some Alewits are unhappy that the unite states trains sunni rebells in Libanion to fight in Syria
Some Afghan want revenge for the civil victims of the (drone) bombings
Some Pakistani want revenge for the civil victims of the (drone) bombings
Some Iraqi want revenge for the civil victims of the (drone) bombings
Some Iraqi who support Assad are unhappy about the pressure the west is doing to avoid weapon transfers to Syria from Iran via Iraq
Some Syrians are unhappy with the military support for the rebells by the USA
Maybe revenge for the murders and secret bombs in Iran near nuclear university professors (highly unlikely)
Maybe FARC rebells are unhappy with the military support of the Columbian goverment by the USA (very highly unlikely)
lots of other things like drone bombings in Yemen, Mexican Drug Lords unhappy with support for mexico by the usa, etc...
In general: a few people died and some people are injured. This is terrible and a crime. But this happens all day in Mexico in drug wars, in Syria in civil war, in the death cells in China, Iran and the USA, in Kongo, in Azawad etc. We have a strange bias to focus the News on the western world. This is quite unhealthy I think.
I recommend to read alternative sources (they are probably not better, no source is good, but different sources are helping to get a broader view). My sources:
http://guardian.co.uk/ <- left western view
http://en.rian.ru/ <- the russian (putin?) view
http://blogs.aljazeera.com/liveblog/Syria <- (Gulf) Arabic view on the situation in Syria, a bit pro saudi arabia / UAE .
indymedia (google it) <- indypendent, left news