Having watched the Coen Bros True Grit last night I've been pondering the reason why Mattie Ross's character was depicted as having complete hatred for Frank James, yet she seemed quite curteous towards Carl Younger, but she called Frank James 'scum'. I've tried researching online and read some articles but it's not solved the question.
I think she makes mention of him being with Quantrill's Partisan Raiders (Bushwhackers). They weren't well-liked by Unionist people in Kansas and had a bad reputation for committing atrocities during the US Civil War. Frank James and his brother Jesse were with that Confederate unit. Cole Younger was a confederate,too, but in the regular army. Mattie was just being a good American citizen.