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.
Trump gave Israeli national defense secrets to Russia, which would love US attention partially diverted from Ukraine, and who happens to be a close ally of Iran (Iran is supplying Russia with weaponry)
Iran is one of the main sponsors of Hamas. Iran has also been under US sanctions of varying degrees for about 45 years, so it would be happy should the US lose face or power.
I'm not saying Iran is responsible, but they were likely middlemen.