The Messenger of Allah (sal Allahu alaihi wa sallam) said: “On the Day of Resurrection, for every betrayer a banner will be raised according to the extent of his betrayal, and no betrayal is greater than that of a ruler.” [Muslim]
Rulers are responsible for the greatest number of people, thus their betrayal is the greatest. A leader’s or ruler’s betrayal is regarded as being so bad because it affects many people, and because he possesses power and authority, so he has no need to resort to betrayal.