Kyosei Tensei

Anezaki Satoru had a precious childhood friend, Ichinose Yuna, who was killed by a gang of rapists. Blinded by rage and thirst for revenge, Satoru gruesomely murdered 15 rapists and cut off their genitals, for which he was sentenced to capital punishment. After his death, Satoru regains consciousness in another world where all men have become mindless beasts. Finding himself lost in this world, he meets some of the struggling survivors fighting the hordes of beasts. What he learns is shocking... Yuna is apparently still alive in this world! Satoru chooses to join the fight in order to find his precious friend.