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I think there are two aspects to a character's moral framework. There are the character's goals (what they want achieve) and the character's scruples (what they will refrain from doing in pursuit o...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/29324 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I think there are two aspects to a character's moral framework. There are the character's goals (what they want achieve) and the character's scruples (what they will refrain from doing in pursuit of these goals). Now, I think that - pretty much by definition - a paragon character's goals will be perfectly selfless and "good". Usually a paragon character will also have scruples, and will have certain behaviours that they just plain consider too evil for _any_ end to justify. This doesn't, however, mean that their moral framework is perfect. It's quite possible that such a character can have overlooked something. There may be things that this character is willing to do in pursuit of their (unquestionably noble) goal that are completely unacceptable to the protagonist. A good example of this, in my view, is in Watchmen, wherein the main villain turns out to be > Ozymandias, a well intentioned superhuman, who believes he can prevent nuclear war and save billions of lives by killing a few million people to fake an alien attack. Up to (and perhaps, from a certain point of view, including) this point, the character is a paragon of virtue. The point is that a character doesn't have to stop having virtuous goals, or stop having scruples in order to become a villain. There just has to come a point where (or a situation in which) their scruples are insufficient to stop them doing unpleasant (or for whatever reason, unacceptable) things in pursuit of their virtuous goals. The character, at least in terms of their intentions, might even be a more noble character than your protagonist, but if the protagonist has scruples that the "virtuous" character doesn't, there will be a reason to oppose them, and if the reader also shares those scruples, there will be a reason for them to get behind the protagonist. Of course, to take a completely different angle, another option is to simply make the protagonist a bad guy (in which case the antagonist can be utterly wonderful, and simply _framed_ as the bad guy by the perspective of the narrative). I'm assuming, though, that this isn't what you have in mind.