Actually, Yang Yi's desire for control wasn't that strong. It's true that he had a motive for rescuing Justin, but the idea of risking his life to save a stranger, someone with no connection whatsoever, was of course completely impossible.
After all, Justin was useful to Yang Yi, worth the risk for him to embark on a rescue mission. However, Justin also had his own plans and a fallback to recover all he had lost, so Yang Yi naturally wouldn't insist on Justin joining the Organization of Water.
Yang Yi understood the saying "a twisted melon never tastes sweet." It was one thing for Justin to join the Organization of Water because he had no other choice, but forcing him would simply turn Justin into an enemy.