Soshite, Mina ni Natta.

In broad daylight, a horrific mass murder occurs, nearly also claiming the life of Mina, a high school student. Miraculously, she survives after undergoing a heart transplant. As Mina begins to settle back into a semblance of normalcy, she becomes alarmed by an overwhelming surge of murderous intent that she's never experienced before. Unbeknownst to her, the heart she received had belonged to Misako Shimizu, the very perpetrator of the mass murder. Mina's parents watch in confusion and fear as their beloved daughter starts transforming into a murderer whose murderous intent never stops...!? (Source: Nihon Bungeisha, translated)