On my point of view it is pretty simple to overthrow an authoritarian government. You You just have to gather a lot of people in the smallest area possible (where the leader is located). Further actions can be made such as restraining the leader in some way.
It is true that that requires collaborative effort by the people and most of them need to be willing to take risks. But are humans selfish enough that they can not realize that is of greater importance the well being of majority?
Authorities are not going to kill millions of people. Are they?
Or if you allow me to rephrase my question in other way. Why are people afraid of the government?