It looks like you're looking for of the manga Rainbow: Nisha Rokubou no Shichinin (often shortened to Rainbow ).
I recently picked up this legendary seinen manga after years of hearing about the equally lauded anime adaptation, and I decided to start from the source material. Let me tell you, Chapter 1, titled "The Dawn of the Brutes" (or similar depending on the translation), is one of the most brutally effective opening chapters I have ever read. rainbow nisha rokubou no shichinin chapter 1 full