The part when a now homicidal Zatsune manages to escape from prison and is now hell bent on killing Miku. Yes you heard that right. She had already given her a brutal beating earlier, and now she plans of MURDERING HER.
And that part in Chapter 7, right before the final showdown, where she is gleefully saying that she'll make sure that Miku suffers as slow and painful a death as possible. Brrrrr.