You are failing to account for what happens when “the system” (whether it is an individual government, or a nuclear nation, or the UN…or anyone else) gets so corrupt that the population recognizes it and starts looking for solutions outside that system.
Vigilante assassinations are what happens. A lone individual is always harder to stop than a group because there is no way for intelligence agencies to intercept inter-group communications that aren’t taking place. There is no group for them to infiltrate. There is no “chatter” to tip anyone off what is about to happen, where it will happen, or how long of a gap there will be before the next event. Why do you think it took so long to catch the Unabomber or the DC sniper? Why was the government totally unprepared to deal with the guy who armored a bulldozer and turned it into a “killdozer” as the media called it? Why was there no warning prior to the guy flying his small plane into the IRS building in Texas? It is because no government can read minds. They can institute social media surveillance, but the ones who are the loudest are the least likely to take action and its the silent ones who are…and the silent ones don’t vent their frustrations online. They hold it in until something drastic happens.
This isn’t anything new. Roman Caesars had to watch for a knife in their back from individuals close to them just as leaders do today.
Wise leaders know that minimizing the number of radically dissatisfied people is to their advantage. Smug leaders who get bold enough to think that they are immune from the effects of an angry public are the ones who end up on the Wikipedia list of assassinated rulers. Hltler was obsessive about his security and it didn’t stop the attempts on his life. The same is true in all societies. Those who abuse power and force and create enemies will eventually slip up at just the wrong moment when one of those enemies is watching…and that is the moment when everything changes.
Did you know that WW1 started with a single assassination ?