Ah... so the good and bad sides are reversed.
Actually everything is pretty gray.
No wonder there was such a big conflict.
One guy wants for all types to live together and now the supposedly good guy wants to exterminate all sinners
What are sins and what is holy? You need to decide what normal is first to tell that. But who decides that? What did we give someone the right to decide it?
Coming in waaaay later, but, I think that's the case. Nightingale probably has the talent of brainwashing anyone she heals, and you can see that Seiya had tear streaks running down his face.
Nightingale must have healed him during one of Seiya's training sessions and used that time to brainwash him, with Seiya only realizing the deception when it was too late.