Barriss was probably one of the earliest Inquisitors to be inducted into the Inquisitorius, maybe days after Order 66.
How long do you think she served as an Inquisitor? I find it hard to believe she wasn't an Inquisitor for a year at minimum (so 18BBY). As long as she would've been put on missions to hunt Rebel Cells and maybe Maul there would be no conflict only protecting the Empire and bringing order to the galaxy.
After the Fourth Sister massacred a village Barris mentioned Inquisitors are meant to bring order not chaos and that bringing fear would turn people against them, in this moment Barriss is obviously conflicted however she just watched a massacre, shouldn't it be less of conflict and more horror if she's still new? It feels as if she's been rationalizing in her mind for a while that what she does is in the name of order and preventing the chaos that rebels spread across the galaxy. It feels like she's not just contemplating the Fourth Sisters actions but her own previous actions as well.
These emotions would only come from serving as an Inquisitor for a while before her current hunt where her inner conflict finally comes to a head when a Jedi she was trying to turn was instead killed by the Fourth Sister.
Maybe before this there's been small moments of conflict due to similar instances of the death of innocents but these sequential events are what finally got to her. So I feel like she may have been an Inquisitor until 17BBY giving her ample time to experience being an Inquisitor and properly turning back to the light.