Earlier quoted context omitted.
Vacuums. Vacuums aren't filled by the populace, they are filled by the strongest of potential successors. Often, one that the original manic was keeping at bay.
That actually leads to an interesting question. What happens if you just keep killing the bad guys? If you kill a dictator, then his evil successor, then his evil successor, does the next potential evil successor realize it's a bad idea and leave a decent guy in charge? Or do you just end up with chaos? Obviously assisination isn't this easy - but if it were?
you're talking about Game Theory. A very interesting question indeed.