I'm sorry but that makes precious little sense. The entire point of omniscient narrative is that it's omniscient, it knows everything. A character's viewpoint may be biased, or misinformed, or limited by his or her knowledge and experience, but the omniscient author's word is the fact of the matter. It's the most direct form of exposition, and it trumps the viewpoint-limited words of a character within the story.
Fact is, sociopathy is a mental illness, and is a condition to be treated by medication and therapy. A clinically diagnosed sociopath isn't legally guilty of actions caused by his condition, the legal insanity defense applies. if Terra was mentally ill, she would be absolved of direct responsibility and could not be jailed, only placed in psychiatric treatment.
The author is quite specific to eliminate any such mitigating circumstances. She is not a victim of mental illness, she's just "insane" without any cause, "through no fault but her own". It's right there in black and white on the page. I do wish you'd argue what was there instead of what you'd like to be there.