Vengeance
It twisted them, changing their views and warping their reality. Told them that they were correct, that the others just wanted to destroy them. Made them and all others think that they were a cursed clan.
It was vengeance, the thirst for revenge and justification that their way was right.
And to few recognised it, to few were able to make their own peace, and realise that the world did not owe them a favour for the sins of their forefathers. That the 'good' the 'peace' and the 'harmony' in the world did not come from vengeance. But rather from acceptance and humility. Their clan did not learn from the sins their ancestors had committed before trying to avenge them, only using them to fuel their hate and anger.
And all Itachi wanted was a change, for his brother to see that peace and true happiness did not come from revenge, but rather from love.
And one day, Sasuke would.
Even if Itachi didn't get to see it.