Tomurai no Kyouhan-sha

One day, his younger brother commits suicide. Seeking the truth behind it, the protagonist Asato logs into his brother’s PC. There, on a certain message board, he finds records detailing the brutal bullying his brother endured. Asato vows to track down those responsible and take revenge.

Then, he receives a phone call from someone claiming to be the administrator of the forum—an invitation to a “Revenge Sharing Group,” where participants help carry out one another’s revenge in exchange for having their own revenge fulfilled.
A fate-bound suspense story in which seven people carry out seven acts of revenge begins!!

(Source: Pocket Magazine, translated)