Why did Bryon and Mark act tough?

Byron and Mark were a product of their environment. Lack of money, coupled with the instinct to survive, often causes people to do things they otherwise would never have done. They live is a world where money is scarce, crime is high, and violence is the norm. Thus, they act tough and try to be tough. For them, this was normal.

“We had played a kid's version of gang fighting called "Civil War," and then later we had got in on the real thing, we fought with chains and we fought barefisted and we fought Socs and we fought other grease gangs. It was a normal childhood.”