Soshite, Mina ni Natta.

A random mass murder takes place in broad daylight... high school girl Mina is a victim, but barely survives after undergoing a heart transplant. Just as peace finally returns for a moment, Mina finds an unprecedented murderous intent rising within her. In fact, the heart transplanted into her belonged to Shimizu Misako, the perpetrator of the random murders! Mina's parents can only watch in bewilderment as their beloved daughter transforms into a murderer, but her murderous intent never stops...!?

(Source: Tokyopop)