Rereading this in preparation for Uzumaki's anime premiere and it just occurred to me that the original Tomie may have been sane and may have been a victim herself.
We've never seen Tomie's origin. I think the very first chapter with her, where she was a student who got murdered and came back, was before Ito made her a full-fledged inhuman thing. I'm pretty sure he had no greater plans at that point in time, so it's definitely not her origin story. But logically she had to come from somewhere.
If she ever was "normal," it would have been long, long ago. The horror of her situation, and her constantly fragmenting will—well, there's no way she'd be left sane at the end. This guy's situation is one of the bleakest fates I've seen in manga given Tomie's immortality. This makes me actually want to see an origin story for her, of a normal but beautiful woman in feudal Japan who got afflicted by some supernatural curse.