“To be greatly and effectively wicked a man needs some virtue.”
— C. S. Lewis, The Screwtape Letters
Unless you’re writing satire or writing about particular periods of history starring a real-life monster, you’re probably taking great pains to avoid writing a two-dimensional cackling villain who does bad deeds for the lulz. (I’ve written before about some strategies for writing a dramatically evil villain who is still believable).
However, it’s very easy to swerve too far in the other direction and move from nuanced character development to actively making excuses for the villain’s behavior and squicking out your audience in the process. This is particularly likely to go wrong in historical fiction, where the insistence that everything must be morally ambiguous necessitates a twisting of fact around situations that have clear heroes and villains. But built-from-scratch villains can also fall victim to this trope (note that I say villain, rather than antagonist. This post concerns true ‘baddies’ rather than a character with positive goals and actions that happen to conflict with the protagonist’s goals and actions).
As I (and other, smarter people) have said before, the villain’s motivation needs to follow some internal logic in order to be believable, even if that logic is based on a faulty premise. But understanding someone’s behavior does not necessitate the narrative minimizing their villainy or offering their motivations as a mitigating factor. A ‘balanced’ or ‘believable’ villain does not need to be sympathetic or a morally ambiguous antivillain; their good qualities do not have to redeem them. In fact, a character can even have a sympathetic or relatable motivation and still terrify the audience with the means they choose to achieve what might seem, at first, like a reasonable end.
Western society has a habit of conflating the possession of socially valued attributes–intelligence, determination, looks, wealth, and so on– with being a good person, and that having positive traits (at least in particular circumstances) means that this person cannot be capable of doing enormous evil. For evidence of this, look at any case where a popular celebrity is credibly accused of some wrongdoing, and a legion of fans react with ‘but they’re so talented, they couldn’t have possibly…’. (Heck, see everyone who defends their friends against founded accusations of antisocial behavior by saying ‘but he’s a good guy’ or ‘she’s always been nice to me.’).
One can’t control audience reactions. That’s part of letting your story out into the world. At the same time, it’s worth being mindful of how you develop your villains. There is a unique tragedy in seeing the potential which a villain threw away when they chose their path, but two much time wallowing in their Dark and Troubled Past is a recipe for romanticizing and excusing their actions. Let the villain be human, conflicted, and have the necessary skills to cause the protagonists real trouble; heck, let them have a sympathetic goal. But make sure the audience also sees that they had an opportunity to do good and turned away, either because they wanted to use ruthless methods to achieve an admirable goal, or because their goal in and of itself is harmful to others.