Sentence:  The reappearance of tyranny [in the 4th century BC] had many reasons... one of the main causes was the development of antagonism between rich and poor; <WSD>tyrants</WSD> came to power exploiting a social and political imbalance within the state. 
Meaning of the word marked with <WSD> tags:  A usurper; one who gains power and rules extralegally, distinguished from kings elevated by election or succession.