"Who will guard the guards themselves?" Who is going to stop them? When they are already of the highest authority in their country/ in the world.

They can bribe or threaten anyone they need to with their wealth and authority. They can control the media so their crimes do not get exposed. Or shift the blame to someone/ something else.

Laws were supposed to stop these kind of things, but they can just change the law to suit their needs, since the laws are created and controlled by people.