tyrant vs ruler

tyrant and ruler both are nouns.

Word	Noun	Adjective	Verb	Adverb
tyrant	Yes	No	No	No
ruler	Yes	No	No	No

As nouns, ruler is a hypernym of tyrant; that is, ruler is a word with a broader meaning than tyrant:

tyrant: in ancient Greece, a ruler who had seized power without legal right to it
ruler: a person who rules or commands

Other hypernyms of tyrant include swayer.

tyrant (noun)	ruler (noun)
a cruel and oppressive dictator	measuring stick consisting of a strip of wood or metal or plastic with a straight edge that is used for drawing straight lines and measuring lengths
any person who exercises power in a cruel way	a person who rules or commands
in ancient Greece, a ruler who had seized power without legal right to it