In chapter 1 Ponyboy Curtis is walking home from the movies when he is jumped by a group of socs. Ponyboy says "I backed up away from the knife, of course, I backed up right into one of them." This shows how violent the relationship between the socs and greasers is.
There's an abandoned church in Windrixville.
When Bob comes by in Randy's mustang to pick up their girls. Two-Bit hands ponyboy a broken pop bottle when it looks like the socs are going to fight. This shows just how fast things can escalate.
Johnny says "I killed him". After killing Bob in the park. This book isn't just about muggings and fights it also includes murder.
Dally hands Ponyboy and Johnny a heater (gun) after Johnny kills the Soc. The characters are willing to use violence to protect each other and themsleves.
Johnny and ponyboy save kids from a burning church showing that the kids are never really safe.
The rumble is an example of pure violence. The fact that they have to specify no weapons making it a "Fair fight" shows just how violent the rumbles sometimes get.