Okay, in the Seventh Sister's first appearance, we saw her mental state as damaged because she acts savage and unstable.
While she was interrogating Ezra, she briefly showed despair over the Jedi Order's fall. Then she came to her senses and showed sadism when asking about Ahsoka. This is due to emotional distress as she herself was a survivor of Order 66. There was something going on in her head for a little while.
In Inquisitor: Rise of the Red Blade, according to Iskat, there was something broken and shattered inside the Seventh Sister and "what was left felt like a blade that ached for blood."
Based on these hints, it would seem the Seventh Sister did NOT become an Inquisitor willingly. What are your thoughts?