Juuseiki Guilstein

A series mass-murders where 15-year-old boys have slaughtered their own families. Iori Takigawa is a 15-year old boy who often dreams that he has become a monster and kills his family. And in those dreams, there is always a girl floating in the air... At school, Iori hears a rumor from Imazeki, the son of the police commissioner, about the truth behind the incidents. It seems that in each case, the 15-year-old boy suddenly transforms into a monster and attacks his family. On his way home that day, Iori and his friend Kikuchi are attacked by a group of delinquents. Suddenly, Kikuchi transforms into a giant monster, fights off the delinquents, and then turns on Iori…

(Source: Shogakukan, translated)