You are correct but I will put it a different way.

North Korea holds power over its people thru the use of force, violence and intimidation. And the world as well as most people think the current government is legitimate. By that logic, anyone or any authority that can take out that government thru the use of force or violence or intimidation would be just as legitimate.

In other words what makes Kim Jong Un more legitimate than if China were to install a puppet leader or if there US were to forcefully remove him and institute a democratic system as in South Korea?

Personally I think you are just as legitimate to attain and hold power providing you use the same rules as the current government used to attain and hold power.