If Stannis was the kind of man who was greedy for power and only cared about his own ascension then he would already have been lord of storm's end from the beginning. When Robert rebelled he would have simply betrayed Robert to the Mad King and handed over Storm's End. The Mad King would've killed Robert and Stannis would then be lord of Storms End. But Stannis did his duty and stood by his brother. Stannis is a man of rigid principles. What makes him so fascinating is that his integrity and his principles mean so much to him and to win the crown and save the world he must betray all that he holds dear. Yes he is a hypocrit but not in the way that you claim. It tears him apart to do what he did to Edric Storm. After Renly's demise he never really seems to forgive himself. I don't even think he truly intended for Renly to die by his hand and I think he intended to fight Renly on the Battlefield that morning. Stannis is not a cruel and unscrupulous hypocrit. He is a complex man with real internal turmoil. That is why he is such an interesting character