Yumiko doesn't meditate. She can't. She's afraid to.
When Yumiko meditates, Yumi's there waiting for her. Yumi's waiting to talk to her, waiting to convince her that it's time she accept both sides of herself instead of insisting on this schismed personality.
Meditation makes Yumi sound rational and Yumiko can't accept that. Yumi is the irrational part of herself who does those horrible things with a sword. Yumiko isn't like that – she's sweet and self-effacing and as harmless as a butterfly.
Yumi exists because Yumiko needs her. Yumiko needs Yumi to be the killer. Yumiko needs Yumi to carry the weight of the evil they do.
If you were to ask the chicken and egg question about this situation, the answer would be that the killer came before Yumi.