Iran is at best a theocracy masquerading as a democracy. The Ayatollah is a dictator for life and approves any 'democratic' candidates. As for why Iran is an enemy it's mostly to do with the hostage crisis, an act of international barbarism. Under the Shah US-Iran relations were actually quite good. The second thing is the idea of destabilizing other countries in the region through exporting the 'islamic revolution'.

https://www.nytimes.com/2017/05/17/world/middleeast/iran-pre...