noun
tyrant
TY-runt/ˈtaɪrənt/
A ruler or person in authority who treats others cruelly or oppressively.
The tyrant rejects the petition while guards block the petitioner.

Etymology
Greek tyrannos described an absolute ruler, especially a usurper, who could nevertheless be just or popular. The title became a reproach, giving tyrant its emphasis on cruelty and oppression.
In context
usurps the throne and becomes a tyrant and is able to torture and put other people to death and enslave them