Why is Scout Finch so angry at her cousin for calling her daddy a "nigger-lover'? I know what nigger-lover means. It's referred to black people.

Certainly this statement is offensive no matter what the time period or context. Unfortunately this term was common among many whites during this time period, especially in the Southern States. Scout however grew up in a home where human dignity was of great importance. The slur was more an attack on Scout's father than on black people. Although Scout is aware this term is objectionable, she takes issue that her father is being belittled.

That's important! :)