In To Kill a Mockingbird, the violence is often alluded to or described after the fact. There are moments when Scout gets into fights at school, but that is just child's play. The real violence in this book concerns adults.
One violent moment is when Atticus kills Tim Johnson, the dog. This is important for a few reasons. It is the first time Scout and Jem see Atticus as something more than a feeble, old academic. Atticus also shows how violence is a last resort. Killing Tim Johnson was necessary to protect others in the town.
The rifle cracked. Tim Johnson leaped, flopped over and crumpled on the sidewalk in a brown-and-white heap. He didn’t know what hit him. (Chapter 10)
Also, notice that Atticus' glasses were destroyed in the process; a subtle indication that this act required a slight change of persona.
Bob Ewell's assault on Mayella is never described; it is only alluded to by Atticus. Bob's history of abusing his children (and possibly Mr. Radley's supposed abuse of Boo) is just understood. Atticus is the one who brings it up. The final violent act is when Bob attacks Jem and Scout on their way home from the Halloween party. There are a few violent descriptions of this attack (Chapter 28), including Jem's arm breaking and Bob reaction to being stabbed.
We were nearly to the road when I felt Jem’s hand leave me, felt him jerk backwards to the ground. More scuffling, and there came a dull crunching sound and Jem screamed.
One’s mind works very slowly at times. Stunned, I stood there dumbly. The scuffling noises were dying; someone wheezed and the night was still again.