Oh, sure, because there is nobody in the D&D world who'd throw over a tyrant. These sort of things happen exactly as frequently as in reality. Nevermind, I don't know, the six adventurers per square meter in every tavern.
Now really, it's silly to say that totalitarianism works the same way it does in our world. In D&D, there are powerful people who sell their swords, murder by the thousands or play the hero and get away with it without trouble. There is magic in the air. Divinations get by every mundane wall. Guilds, houses and organizations scheme endlessly. And all of them will have adventurers only their payroll if they know what's good for them. A king needs good publicity at least among the people of his kind. Respect from his peers, his church, his family, those of similar alignment, whichever. One wrong step in the wrong direction, only one more enemy than what his own adventurers can take on, and he's as good as dead. None will see them coming.
The king may not realize that(my whole 'only laid back people can think of this crap' theory is responsible for this assumption), but clerics have positive Wisdom modifiers. They will keep the idiot safe at least from themselves. Unless his demise could turn out to be useful, naturally. Yay for intrigue.