Curley beat up Bryon after Mark cut his sister Angela’s hair.
Mark got Angela drunk and cut her hair one night in revenge for her starting a fight at a dance. Curtis and his brother and some other boys later attack Byron in revenge of the hair-cutting.
[They] pinned my arms, and held me while Tim and Curly took turns punching me. I passed out finally, but not as soon as I had hoped I would. (ch 8, p. 72)
Byron has realized that the constant revenge is a vicious cycle, and ironically he tells Mark not to get even for Curley beating him up, because there has been too much violence already.
Byron is sick of the violence, but it is the way things are in their neighborhood. Everyone runs with a gang or is affiliated with one in some way, and it is a matter of honor to get revenge for any judged disrespect or abuse.