I know what the Sumner/Brooks affair is, but how did it play a role in leading up to the civil war?

Answer 1

As Sumner was a liar, once he got beat up, he got a lot of attention. In the North, abolitionists made him a hero, whilst in the South, Brooks was an instant hero.
I suppose it demonstrated on both of their parts, how lack of civility can inflame passions that can get out of control.

Answer 2

Read this article. It will give you an idea of the antipathy on both sides prior to the Civil War.

http://en.wikipedia.org/wiki/Charles_Sumner#Antebellum_career_and_attack_by_Preston_Brooks