Darumi is one of the few people who saw through Takumi and recognizes that Takumi only sees women as fuckable pieces of meat, not as real people.
She knows sooner or later Takumi will succumb to his urges and start raping the girls. Out of selfhatred and desire to do a good thing, Darumi offers herself as a sacrifical lamb, a bait, to keep Takumi away from the innocent girls.
It wouldnt have been the first time she got raped, but she wants others to not experience the pain.