Hill felt that there might originally have been an existence that Styx found useful but did not quite trust.
This is quite normal.
Toril has many of those kinds of characters, who have a bit of a bottom line and are unwilling to commit heinous acts in other worlds, but themselves have possibly cooperated with devils and dealt with demons.
Although All Gods would also cooperate with such characters, entities like Styx would likely be more stringent in their judgment.
In desperate situations, they might throw a seductive glance, but if there's another opportunity, they definitely won't choose to cooperate with them.
Such people, although seemingly special, are actually quite common in Toril... Even figures like the Deepwater City's Mad Mage don't have much of a reputation among these people.
Not to mention Aber's natives, some strong ones among them are quite willing to wander across the Multiverse.
