Kimi ga Kemono ni Naru Mae ni

At the end of the year, a poison gas terror attack takes place at a terminal station in Tokyo. 666 people die or are injured in the attack.The terror attack's number of victims is related to the "number of the beast" and the case is given the name "The Beast." The person who carried out the attack is a 25-year-old popular actress, Kotone Kido, who died at the terminal station. Kotone Kido had a childhood friend named Hajime Kanzaki. They were not blood-related, but they were very close like a brother and sister. Now, Hajime Kanzaki is 33-years-old and works as an undertaker. He wants to find out why his childhood friend committed the mass murder, who made her do that, and was there a chance she could have been stopped. While he investigates, he is killed by someone. At that time, he travels back in time and meets Kotone Kido again.
(Source: AsianWiki)
~~ Adapted from the manga series "From the Dust." (君が獣になる前に) by Sano Takashi (さの隆).