I'm wondering whether the word dictator should be capitalized.
Is it just an adjective and not an (official) title?
It depends on context and preference. If used as a noun, it's not capitalised:
He was the dictator of Sicily.
If used as a title, it ought to be:
He proclaimed himself Dictator of Sicily.
And also if used as an honourific:
He was called Dictator Garibaldi.
Nowadays, Dictator doesn't have any positive connotations so as @Jon mentions, it would be hard to find it capitalized correctly. In the past for example, Simón Bolívar, a South American General, was proclaimed Dictator of Peru.