That creates a slippery slope. When does the UN draw the line that a government has revoked its sovereignty? How many people have to die? How can it be justified that only if x number of people die, then we will intervene? Additionally, as soon as the UN gets involved in a civil war or dictatorship and has deemed the government no longer sovereign, then who is in charge? Is the UN going to set up a new government and country in the aftermath? That is a large commitment that such a large organization may not be able to execute no matter how ideal.