Ezar Vorbarra needs a man who won't do what he did. He needs a man who, when his friends and enemies come to him on their knees, will refuse their oaths. Ezar needs a man who knows how to obey his Emperor no matter the cost.
Piotr trained his apprentices well, and Ezar has repaid him by crafting Piotr's son into the man he needs. Piotr raised his son to be a Count. Ezar molded him into being his contingency.
Because Aral has one advantage to his father, and to Ezar himself: he doesn't know how to be a traitor.