Tomurai no Kyouhansha

One day, Asato's brother committed suicide. Upon learning that it was due to bullying at school, Asato sets out to find and punish the bullies. Then, he receives a phone call from someone claiming to be the administrator of a "troubled discussion board". It turns out to be an invitation to the "Revenge Sharing Society," where you help with others' revenge in exchange for assistance with your own.