To this point, everyone has underestimated the heroine. She survived a near-drowning in infancy when her mother jumped off a bridge, but bears a scar as a constant reminder of this primal injustice. She grows up to be a high-wire artist but has grown sick of all the wolf whistles by the time the story proper starts. Shin's scheme looks like the easy path to a new and better life, but after the great betrayal the only path left to her is the path of revenge.
On the island, Ohyaku becomes the plaything of a privileged convict and his wife, a tattoo artist (Yuriko Mishima) who grows obsessed with the heroine's smooth skin. She aches to tattoo and otherwise make Ohyaku her own. Ohyaku appears to give in, and rather likes the idea of a big nasty demon tattoo on her back, but for her the non-poisonous seductress is just one of many people she manipulates and leaves in the dust on the way back to the mainland. There, her ultimate revenge will be twofold. With some gangster friends she'll rob not merely a gold shipment, but the mint itself -- and then she'll kill the bureaucratic bastard who ruined her life the same way he had Shin killed.
Female Demon Ohyaku has an oldschool attitude toward revenge from which modern American pop culture has grown alienated. We've long understood that revenge can make a monster of you, the end justifying all means and all manner of mistreatment of not just the guilty but the innocent. Nowadays, we like to catch someone on the brink of taking revenge and tell them, "This is not you" or "You're better than this." I don't know if Japanese attitudes have changed in a similar way over the last 50 years, but in this picture, set sometime during the 19th century, when Ohyaku tells her friends what she's going to do to her past tormentors, their response basically is, "You go, girl!" Some modern viewers may expect her to stop short, to decide that the bad guy isn't worth what Ohyaku presumably is doing to her own soul or karma, but let me assure you that she does not stop short. There's something both awful and awesome about that that may be lost on those whose moral hedonism is so absolute that they can't even imagine anyone actually deserving to be killed. Movies like Ohyaku allow a vicarious release from the passivity such absolutism may encourage, without necessarily convincing anyone to turn killer. Ohyaku's revenge may seem self-indulgent to some viewers, but it also lends the film a kind of gravitas lacking in today's more common appeals to "hope," regardless of whether you think the heroine right or wrong. One can assume that identifying her as a "demon" makes Yoshihiro Ishikawa's film something other than an unambiguous endorsement of Ohyaku's revenge, but the fact that she is a heroine probably tells you as much about the world she lives in, or the world as Ishikawa saw it, as it tells you about Ohyaku herself.