Hate to say it, but stuff like this isn't going to stop. We could spend twice as much on counter terrorism, military, etc, and these senseless tragedies will continue to happen. Terrorism was around in the 1600's with Guy Fawkes, and it will be around 400 years from now. I wish I had an answer as to why these things happen, but like everyone else, I don't. My message to anyone who would ever be involved in terrorism/murder of innocents... KILL YOURSELF. Don't hurt anyone because you're screwed up, angry, crazy, insane, hungry for revenge. Just snuff yourself and move on to a place where perhaps you'll be happier.
I'm really glad I live in a country where overall we pull together and help when these things happen. This is the greatest country in history, and these attacks will never take that away.