WordNet sense 1 (of or relating to or associated with or resembling a dictatorship):

tyrannic

WordNet sense 2 (marked by unjust severity or arbitrary behavior):

oppressive, tyrannous

WordNet sense 3 (tending to domineer):

domineering

WordNet sense 4 (characteristic of an absolute ruler or absolute rule; having absolute sovereignty):

authoritarian, autocratic, despotic, dictatorial

WordNet sense 5 (not in agreement with or according to democratic doctrine or practice or ideals):

undemocratic