That seems like a weird answer to ask. He’s just a video game character. But for the sake of discussion, consider the following.
When defending Fabul, Kain betrays Cecil and takes the crystal for himself. At this point, he’s working with Golbez. We don’t understand why yet. When we fight Golbez at the Tower of Zot, we break his control over Kain. Cecil knows immediately what happened:
Cecil: Golbez held sway over our mind. None of this was your doing.
Kain: But…I was conscious of everything. Rosa, she…I-
Kain then explains that not everything was Golbez’s doing:
Kain: Forgive me, Rosa. Not all of what I did was because of Golbez’s spell. I just wanted to keep you…to keep you by my side.
Which is somewhat of a shock to us. They never really establish, or even hint, that Kain loves Rosa. It could be possible since Rosa is the most beautiful woman in Baron. Who wouldn’t be jealous? But what if
Kain joins our party but sometime after the Sealed Cavern, Golbez takes control of Kain again. Near the end of the story, we realize even Golbez was under someone’s control. As Rosa explains:
Rosa: Golbez broke free as well. That’s why you’re free. None of this is your fault.
Rosa: A Lunarian named Zemus was making use of Golbez’s as a means to control him.
I wonder if Zeromus somehow manipulated the game’s files to alter Kain and Golbez’s minds. Zeromus would give Kain the motive of jealousy to fight alongside Golbez. Golbez’s motive would be to fulfill Zeromus goal as a Lunerian. But in this sense, Zeromus is trying to take Kain from our party to sabotage our characters. Why Kain? Because his code must be susceptible to manipulation.
Of course, this isn’t possible. That’s not how the game’s code work. Obviously, each of the characters has to fulfill their roles according to the script. And the script determines their actions and the events we see. They wouldn’t have done otherwise if Zeromus didn’t control Kain. Maybe Zeromus doesn’t understand he’s in a game. The script gave him commands to carry out, and he does so with full awareness of the plan. Couldn’t all our characters have self-awareness this way then? It doesn’t quite work this.
Still, if we deleted Zeromus’ files, none of this would happen.