Would be true if leaders didn't commonly justify their crimes by misconstruing and manipulating ideologies. E.g. the Crusades, Cold War and Holocaust started as "harmless" ideologies
Tyrants use ideology to rule masses. Take Fidel Castro who's originally opposed to communism but changed when he saw opportunity. But on itself ideology does nothing, its ink on paper.
That might be true for most, but in the past ideologies that push the harm or censorship of citizens have been incredibly destructive. E.g. book burning, eugenics, slavery, fascism...
It's not necessary to ban their study, but populations should 1st be taught how they caused societal backslides, stunted long-term growth and widespread, recurring violence in the past