Although Fengo secretly felt that Asmodeus was taking advantage, after all, he wasn't an original spawn of Hell... if it were any devil, they would have unhesitatingly assimilated with Hell.
This doesn't mean that the Lord of Hell would lose his personal will, he just needs to completely consider matters from Hell's perspective and absolutely cannot violate the fundamental Lawful Evil faction of Hell.
However, for Asmodeus, who came out of Heavenly Mountain and was backstabbed by the God of Order he once followed, having reservations is understandable.
Even the Will of Hell didn't force Asmodeus to merge with it, and it even allowed him to maintain a deep connection with Hell indirectly through the Princess of Hell, so Fengo won't meddle either.
As for whether the Will of Hell secretly had Glaysia do things that left Asmodeus with no choice, Fengo wouldn't look into it, no, he wouldn't even think about it.
He just needed to be a diligent Hell Prosecutor.
