Look around the world and ask yourself: why does it mainly happen in the US?
Every case is different but there are 2 common origins:
The need for the perpetrator to get recognition, even if bad or after his death, but at least recognition!
Your country's imbecile an excessive gun tolerance.
Same in Iraq, BTW....there are so many weapons circulating and a gun is considered a symbol of freedom...very tempting to use it.