Juujika no Rokunin

"Is it okay to kill people?" A dangerous revenge thriller from Besatsu Shonen Magazine newcomer Shirou Nakatake. If someone repents, let him go and do nothing. He should be given a chance to be reborn.”
"Grandpa, I hope he doesn't reincarnate himself." In order to defeat some inhuman monsters, the boy also became some inhuman.
Sixth grader Shin Yuroma is named "Experimental Body: A" by five of his classmates and lives in Hell. His only peace is with his brother, who loves him, and his parents, who protect him. Until five monsters kill his family.
When he finally lost everything and faced the true Hell, a dark "desire" arose within Shun. He was reborn under the tutelage of his grandfather, who served in a secret unit during World War II. Now, four years later, he faces the enemy of his destiny. "I will not let anyone stand in the way of my revenge."
Labeled "Experiment A" by five of his classmates and bullied in sixth grade, Shun Aroma hangs by a thread. When his goons targeted his family, leaving him with no one but his grandfather, that was the final blow—Aroma was pretty damaged. Determined to get revenge on his enemies, Aroma pleads with his grandfather, a World War II veteran, to lead him on a path of revenge.
Four years later, Aroma completes his rigorous training and embarks on a brutal journey of violence and revenge to hunt down the five monsters who made his school life a living hell.