imperialist is a noun and imperialistic is an adjective: As a prime minister he was an imperialist. He had imperialistic policies

Words ending in -ism are schools, ideas and beliefs. They are nouns: Capitalism, communism, socialism, liberalism, federalism Those ending in -ist are people. They are nouns: Unionist, maosit, terrorist The ones ending in -istic are adjectives used with other nouns: Imperialistic policies, futuristic ideas, kabbalistic teachings

Thank you very much Steven!