Plato diagnosed tyranny as first and foremost an affliction of the soul. Socrates in Plato’s Republic characterizes the tyrannical soul by a malformed desire that strongly resists any kind of balanced consideration of other factors. This kind of desire wants its way immediately and unconditionally.
The tyrannical soul wants a kind of unquestioning recognition from others, without reciprocally recognizing them. This kind of attitude represents the opposite end of the spectrum from what Aristotle called magnanimity or great-souledness; rather, it is characteristic of the attitude of Mastery denounced by Hegel. Unfortunately, modern egoism, with its emphasis on a narrow kind of self, tends to devolve in this direction. (See also Freedom Without Sovereignty.)
While a tyrannical soul may be an Aristotelian cause of particular unjust acts, this does not mean that injustice as a whole is reducible to matters of individual character. Injustice is not just caused by the bad acts of individuals, but also often involves institutions and social structure, which have their persistence in part from a kind of materiality of their own.