Answer: It is impossible for the King of Mordor to have the consent of the people because the monarchy is not a freely formed government in which the people have a say in how it operates.
Jean-Jacques Rousseau was a Genevan philosopher, writer and composer. He is particularly well-known for his political philosophy in Discourse on Inequality and The Social Contract, and for his concept of the "general will."
If Rousseau were to analyze the case of the King of Mordor, he would say that it is not possible for the king to have the consent of the people, as he claims. Consent is not defined by whether the actions of the ruler are good for maintaining order or not. Consent is only defined by whether the people have chosen him to represent their interests. As they have not, and they are also not permitted to participate in the political process, the king cannot claim to have the consent of the people.
