So, I’ve mentioned before that Marietta Edgecombe’s treatment in HP bothers me, and gotten several inbox messages basically asking “Why does it bug you so much?” This answer is probably long-overdue, and like everything I write, will likely be just plain long.
What happened to Marietta again?
Marietta Edgecombe was Cho Chang’s best friend, who Cho cajoled into joining the DA. Marietta’s mother, Madam Edgecombe, worked for the Fudge Ministry, and the knowledge that they were disobeying the Ministry to follow Harry (who Marietta distrusted) eventually got to Marietta, who informed Umbridge about the DA. That allowed the Inquisitorial Squad to break up a DA meeting, capture Harry, and almost force Harry’s expulsion - and it did lead to Dumbledore’s departure from the school.
However, Hermione had jinxed the parchment that DA members had signed when they joined up, and when Marietta broke their code of silence, a jinx came into effect: large purple pustules spelling out “SNEAK” covered Marietta’s face. Those pustules faded a little with time, but permanently disfigured her face: after getting out of the hospital wing, she wears a balaclava for the rest of OotP, and the marks are still vislble even under a thick layer of makeup in HBP.
Marietta is not seen in DH, but in 2007 when JKR was asked about what happened to Marietta and if the scars ever went away, she answered that it faded a little eventually, but still left scars, and punctuated that with “I loathe a traitor!" That implies that Marietta was left permanently scarred by the experience - and not only that, but demonstrates JKR’s explicit approval of the scarring: the perma-scarring is an authorial decision that reflects JKR’s dislike for Marietta and basically holds up the punishment as right and just, something that the author applauds - and that, by extension, we should applaud as well.
JKR and Traitors
I do think that betrayal - particularly betrayal of our heroes - is something that JKR loathes; this may be a part of why Gryffs often try to aggregate Hufflepuff loyalty onto their House traits, because JKR the consummate Gryff values it so highly and deals so harshly with the traitor characters. Characters who are traitors - specifically who betray the heroes of the series - aren’t afforded the sort of moral complexity given to other characters; they’re indelibly marked as bad, not afforded the sympathy given to other complex characters (including those who have betrayed evil characters: Snape is vastly better-developed than Pettigrew, and fandom at large far more sympathetic to the former than the latter.) Pettigrew betrays the Marauders, and is afforded little character development, less real role in the plot, and shunted aside quickly with an unsatisfactory death in DH; Marietta betrays the DA, is permanently scarred, and is generally never afford the sympathy or complexity that she deserves.