Democracies are not a guarantee of civil rights and easily turn into authoritarian and repressive regimes.
Usually only after they stop being democracies. It's rather difficult to repress people who can fire you if they don't like the job you're doing.
But in principle I agree, democracy on its own doesn't guarantee morality. There's such thing as the tyranny of the majority.
Democracy is a morally superior system of government, because it's fundamentally premised on a moral idea; that governments "derive their just powers from the consent of the governed". Dictatorships and aristocracies can make no such claim.