At this point, he is our inside agent, but given his fascination and desire to learn Hive Arcana, along with the fact that he agrees with the philosophies of the Hive, it's only a matter of time before he turns on us. It won't be quick. Even if he against all odds gets the mantle of Taken King, he would be out ally until he was confident that he could beat us. He would then likely turn against us as his own free agent.