It's freking strange how i can't hate her despite she invading homes, raping, cutting and slowly and painfully killing women, that's Takechi Otoya for you. It's indeed really interesting the contrast between the authors art styles and mood. From my point of view, like others have said, Takechi doesn't want to kill Shienna not only because she is not her type, but also because she's pure hearted, it seems like she wanted to wait until Shienna grows up into a pretty lady(and loose her pure heart?) to be able to kill her, if you think about it, it can be a form of normal and true love/affection from Takechi, since she had no mercy on Haru(who i can't think not being pure hearted as well) but spared Shienna