Kill Blue, Vol. 1 by Tadatoshi Fujimaki

[Viz] Hardened hit man Juzo Ogami is in for the mission of his esteemed career…surviving life as an average middle school student! Juzo Ogami has never led a normal life. He was picked up by a criminal organization as a child and grew up to become a legendary hit man. But everything changes when he’s bitten by a mysterious insect and becomes a teenager again! Can this battle-hardened assassin handle the rigors of middle school life?! Juzo Ogami’s a legend. Even among hit men, his name sends shivers down spines. There’s never been a job he couldn’t handle—that is, until the day he wakes up as a teenager! But how has his body transformed? To find out, he’ll have to infiltrate the one place he thought he’d never find himself in again…school!