Well, without having read 'the scum of good and evil extra story', it's hard to say precisely. However, extra stories often offer additional perspectives. Maybe it focuses on a particular character from the main story who has a more complex relationship with the concepts of good and evil. It could show how this character's actions in the extra story are influenced by their own moral compass, which might be different from what is considered the norm in the story's world.
Since I don't have the details of 'the scum of good and evil extra story', we can only make some educated guesses. It might explore the grey areas between good and evil. For example, there could be a situation where a character's actions seem evil on the surface but are actually driven by good intentions, or vice versa. The extra story could also introduce new characters that further complicate the understanding of good and evil in the story's universe. It could be about a journey of self - discovery for one of the characters in relation to their moral standing, perhaps they start off believing they are on the side of good but through the events of the extra story, they realize that things are not as black and white as they thought.