Shizuka is deeply hurt and unhappy person. Considering that any reaction is not abnormal in that case. After years of pain anything goes, just to end the suffering. In that case it looks like she thought about killing either the source of the pain or herself.
Honestly watching this show makes me feel so frustrated. I feel like there’s so much you could do to end (relatively peacefully) this suffering but nobody’s taking an action. I understand this happens in many cases but still… We’re on the bad route and I feel like suicide wasn’t the worst of them…