At the beginning of chapter 1, Ponyboy is walking home from the movies and gets jumped. In the text, the Socs say to Pony, "Why don't we give you a haircut right under the chin." They hold a knife to his throat and beat him up pretty badly. This shows how brutal the Socs can be.
In Chapter 3 when Darry hits Ponyboy, it makes Ponyboy really sad and makes him want to run away. Darry says on page 50, "POnyboy I didn't mean to!" but Pony boy just kept running.
In chapter 4 when Johnny kills Bob it's a very clear thing that it really shocks Ponyboy and Johnny, on page 57 Ponyboy says, "Johnny their gonna give you the electric chair!"
In chapter 4 Dally gives Pony and Johnny a gun which shows how they have to protect themselves. On page 60 it says, "...he handed us a gun and a roll of bills..."
In chapter 12 when Two-Bit, Steve, and Pony are at the gas station getting sodas, some socs approach Ponyboy. He casually busts his pop bottle and is ready to use it but doesn't have to because they run away. It says on page 171, "I didn't feel anything..."
The rumble is a key point of violence because it's an organized fight between the Socs and the greasers which frankly it makes things worse. On page 142 Darry says, "I'll take on anyone."