Devil 17 - Hokago no Kusenshi

Devil 17 - Hokago no Kusenshi is a manga series by Gouya Daisuke and Saijyo Shinji.

Plot
In the midst of a terrorist attack that kills all his classmates, Tohru Kuroe knows the chances of seeing yet another woman die makes Tohru pick up an abandoned gun - The beast awakens; taking over. When Tohru comes to his senses, he finds he is now a killer. Finding out he was genetically engineered to be a killer, his codename is D-17, Tohru continues his 'normal' life amidst challengers threatening his life and those around him.