dictator vs ruler

dictator and ruler both are nouns.

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

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

Other hypernyms of dictator include swayer.

dictator (noun)	ruler (noun)
a ruler who is unconstrained by law	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
a person who behaves in a tyrannical manner	a person who rules or commands
a speaker who dictates to a secretary or a recording machine