Killer Peter 

After devoting his life to the crime organization, Glory Club, Peter retires his status as a legendary killer and spends his remaining days running a quiet used bookstore. But after being ambushed and nearly killed one day by the very organization he dedicated his life to, Peter finds his body magically reverted back to its teenage years. Now that Peter was back to his peak physique, he sets out to settle the score with Glory Club. (Source: WEBTOON)