Athetos is 100% a good guy.
He tells us that he can't tell the truth, because Rusalki will then activate a killswitch. What does it tell us? That we would join him if we knew how things really were. That he wants us to survive. That he fakes the fight to begin with, because he couldve easily taken us out by "provoking" Rusalki.
We can say with confidence that it is so, because Rusalki blatantly lie and exploit us throughout the game to the very end, not to mention their previous lore.