tyrant

As a noun tyrant

is (historic|ancient greece) a usurper; one who gains power and rules extralegally, distinguished from kings elevated by election or succession.

As an adjective tyrant

is (uncommon) tyrannical, tyrannous; like, characteristic of, or in the manner of a tyrant.

As a verb tyrant

is (obsolete) to act like a tyrant; to be tyrannical.

As a proper noun robespierre is

the french revolutionary leader , 1758-1794 often used as a byword for a murderous demagogue or tyrant.