The situation that Nagisa finds herself in is comparable to the concept of original sin being passed along bloodlines and creating a totalized class with blood guilt. Reincarnation, on the other hand, isn't so much like a Sexual Transmitted Disease (like Christian sin), because the sin is infused into 'soul' of the individual caught in the karma-tic system. What I find particularly interesting is that even though Nagisa entered into a life with amnesia in relation to past-life's sins, the ghost still persecutes her as if she existed totally as the individual that killed them. In the metaphysical anything can be justified. But let’s saw into this psychoanalytically.
We enter the film with a mostly clueless and submissive woman with self-esteem problems. As the movie progresses though achieving her role in the movie and finding her place in the world the ghosts interject themselves into her life. Scared by the several interventions, her mind seeks to contextualize the trauma, not finding grounding in her lived reality she pushes beyond it, and incorporates her spiritual beliefs as a frame of reference. Taking a step back, what might an average Japanese Buddhist interpret The Exorcist? Of course, they wouldn't jump at the Christian theological framing the film uses, but their own lived reality. The demon, then becomes not some roaming transcendental evil that possesses the body of a child, but a previous life fighting for a fully realized embodiment.