Ferdinand was planning to join forces with Jethro to lure out Hagel and kill him.
Should he let Deyo kill Hagel, or sell the information for some money?
Currently, Yang Yi and Jethro had a good relationship, which went beyond that of employer and bodyguard; they had a good personal relationship, but Yang Yi was a spy.
Spy is the more polite term; undercover agent sounds decent too. However, the uglier terms are mole, traitor, or double-crosser.
He didn't want to betray Jethro, but whether it was his own identity and profession, his goals, or the overall interests of the Organization of Water, Yang Yi had to betray Jethro, and that caused him great pain.
Despite the pain and guilt, Yang Yi wouldn't confide in anyone, nor would he hesitate about what he had to do, because he was a spy.
So, Yang Yi turned around and sold Jethro out.