One of the reasons that the Harry Potter series is facinating, why I have invested the time in this site, is because it raises, but does not answer, so many interesting questions. A number of those questions center around, tangent off of, or could be rephrased as, questions of good versus evil. This is one such.
A number of fan fiction authors trying to make sense of the world of magic, struggle with the nature of good and evil. Many of them do so either inadequately grounded in Christian theology, entirely ignorant of that framework, or even conciously rejecting it as a basis to answer the questions they are struggling with. They thus come up with a wide variety of unworkable answers. These range from the ludicrous to the maddening, from the amusing to infuriating, and everything in between.
Quite a few authors have proposed some variation of the idea that there is a difference between the concepts of “good” and “evil” and the concepts of “light” and “dark.” These authors are purposefully playing with the ideas of “dark magic” that the books themselves largely avoid with very few exceptions (the creation of a horcrux or an inferi are the two most notable exceptions).
I understand what these authors are struggling with. They are attempting to make sense of the facts such as Lupin being a “Dark” creature - a werewolf - but being a “Good” person. The problem is that both the authors struggling with this, and the in-world characters labeling [werewolves] as “Dark” lack a proper understanding of Evil and what it means to cooperate with evil. This is a difficult topic, and a Google Search will bring up quite a few articles attempting to explain it to people honestly confused on the topic. Perhaps the best simple resource I have seen on this is a chart that the Archdiocese of Philadelphia had up for a while, but which is now only available on the Internet Archive.
Taking the case of Lupin, while he is a werewolf, it is not by his own choice, nor does he enjoy being a werewolf. His participation in the evil he may commit, so long as he takes reasonable precations, is Material, not Formal. It would however, be judged Immediate, in that it would not occur if if he had taken reasonable precations. Note that I am not talking about his transformation. I am talking about situations like that which occurs in Book 3 where he, under the control of the werewolf curse, attacks someone (thankfully in Book 3 they escape). Thus he would be judged guilty, not of being a werewolf, but of attacking someone while transformed.
To make the distinction clear, if you take the average case of a full moon, Lupin transforms, but no one is attacked. No evil act occurs, and there is no act to evaluation Lupin’s cooperation with against. Thus while he transformed in both cases, he is guilty of an evil act only in the one. This must be contrasted with the offical view of [werewolves]. Rather than judging the attacks by [werewolves] “dark”, the official view is that they themselves are “dark.” This official view is false. It is the act which is evil, not the person. Unfortunately Lupin himself has internalized this view (“I am a werewolf” versus “I have lycanthropy”). I am fully aware there are those who use these language shifts more to shut down conversation, or to deamonize those who misuse language, typically out of poor habits rather than actual ill intent. Despite this, there is a truth to the insistance, and Lupin’s case highlights this.
In short, there is, properly understood, no distinction between “good” and “light”, conversely “evil” and “dark” but there are *very really distinctions between what people claim to be “light” and “dark” and what really are. We need the courage to say, plainly, that someone, even some entire cultures, are quite simply wrong, or we lose the ability to speak clearly about this distinction ourselves.