Sadly, I would even go a step farther than that. Even preventing the actual acts of terror is extremely hard to the point of being impossible to stop every mass killing. Sometimes good intel can stop them, but just as much it seems when they do fail it's because of pure luck or gross incompetence by the terrorist. That's not always going to be in our favor. There are just too many soft targets that are too easy to kill, and it will always be that way as there is nothing realistically we can do to protect ourselves from people that determined to kill others. It's happened now. It will happen again in the future. Totally altering our society in a futile attempt to do so would just hurt us, and us becoming just as bad as them for the sake of vengeance or false justice just hurts the world that much more.
It's also bad the terrorist don't realize they aren't any closer to winning than they were before. Scaring people and killing a few hundred isn't going win a war one wants to wage against a gigantic portion of the planet. It just pisses more people off against them because they want to kill a bunch of innocent people in vain.
True. It's like they say, the terrorists only need to be right one time while we need to be right 100% of the time