Hey check it out, this guy hasn't heard of Franz Ferdinand! /s
As to not be rude or dismissive: the assassination of Archduke Franz Ferdinand is generally credited with the start of World War I. The implication being that a world war has been started over a seemingly small event that had tumultuous consequences thereafter. The further implication being that we may already be past a Franz Ferdinand assassination type event and may be worryingly headlong into a path to large scale warfare once again.
Because the world is interconnected a lot more than it has ever been before. A single shooting in a very minor nation went from a terrorist attack to a full world war. The world is in a very precipitous state ATM. Many countries are looking for opportunities to take advantage of any weaknesses in world order to subjugate their neighbours. Now is not a good time to go gun totting frivolously.