We see violence first off with Oskar's bruises, and how he punishes himself. We also violence in the "Hamlet" scene, when Oskar imagines that he is crushing Jimmy's face in.
Another display of violence is on pg. 145-146 when he is daydreaming of killing Jimmy Snyder with his skull costume.
"There were things I wanted to tell him. But I knew they would hurt him. So I buried them, and let them hurt me." (181)
Jimmy Snyder pushed my shoulder and said, "Say your mom's a whore." I said, "Your mom's a whore." He said "Say your mom's a whore." I said "Your mom's a whore.""Say 'My' 'mom' 'is a whore.'" "Your mom is a whore." Matt and Dave and Steve and Jake were cracking up, but Jimmy was getting really, really angry. He raised a fist and said, "Prepare to die." I looked around for a teacher, but I didn't see any. "My mom's a whore," I said. (192)
"I counted them last night in the mirror and there were forty-one. Some of them have gotten big, but most of them are small. I don't put them there for here, but still I want her to ask me how I got them (even though she probaly nows), and to feel sorry for me..." (172)