Mutiny

As I recently noted ("What legitimates the state?"), gov't is ultimately an honor system. It only works if people agree to follow orders. If they stop taking orders from others, it breaks down.

This is why regicide is so destabilizing. Consider how many Roman emperors were assassinated. The first assassination is the hardest. After that it's easy. Once the Roman army crosses that line, once the Praetorian guard discovers that it can knock off the emperor with impunity, then all inhibition is lost. The assassin wasn't struck dead by a thunderbolt. 

Of course, promotion by assassination is a two-edged sword. He who ascends the throne by killing the incumbent must forever watch his own back. There will be a line of equally ambitious, equally ruthless men who are eager to emulate his example. 

By the same token, that's why mutiny is so destabilizing. That's why, historically, mutineers were severely punished. It exposes the arbitrary character of the honor system. As soon as a few soldiers or sailors feel free to disobey orders or overthrow their superior officers, that has a domino effect. 

Likewise, that's why the Soviet Empire imploded so fast. It had no popular legitimacy. Only fear kept the regime in power. 

An illegitimate regime cannot survive indefinitely. There comes a point when people refuse to follow orders.