>>11827778>>11827384Getting a bit below the surface in psiv, you see that Zio is no better than the party for the most part. It's even possible to argue that he's more enlightened than the party at that point by far.
The great light didn't actually communicate with its chosen ones until rykros shows up, but it preemptively set its followers up to basically manipulate them into doing its bidding while it fucked off to do shit in space. Even Rykros was staffed by its servants, basically the good equivalents to Dark Force.
The profound darkness directly chose its champions, including Zio, and seemed to be completely honest with them from the start. The one thing against this is that Zio only ever mentions Dark Force, but he also had no reason to do over-exposition, and even then, since DF is just a piece of PD it doesn't change much anyway.
Going from there, Zio also had no reason to explain anything. He did not know how much the party was aware of, and the less they know, the better. Doing a classic evil villain monologue was literally the correct move.
All this is to say that in his eyes, all he did was get rid of another unaware goon for the opposing side. Why'd he do it? Because you weren't just random adventurers, he knew you were the enemy even if you didn't.
Bonus: We're told that the entire system was created just to seal the darkness after it lost to the light. In other words nothing actually tells us the darkness was evil before it got imprisoned for eternity, which would probably make anyone salty as fuck. These ideas of PD being more netural were explored later in pso2, but that was revealed to be a fully separate universe.