Madara is Darker Sasuke in the same way that Obito is Dark Naruto.
They both are proud Uchiha, both had best friends who weren't Uchiha that want to change them, both betrayed their village for what they thought was in the best interests of the clan, both are motivated by avenging a brother who was very precious to them, both are usually calm but absolutely terrifying when not calm, and both don't like to admit their mistakes
Their development is very parallel. I can only hope that Sasuke surpasses Madara by admitting his mistake and accepting Naruto's friendship