How To Write Sympathetic Antagonists Without Endorsing Or Excusing Their Actions, & Without Making Your Protagonists Seem Heartless

Build up sympathy and encourage empathy for those who are impacted by your antagonists' harmful or hurtful actions. Nothing says "what just happened here is not okay!" like showing how it harms an undeserving person. Humanizing someone that one of your antagonists will mistreat or harm, building that character up as sympathetic, and then showing how that character suffers as a result of the antagonist's actions can be one of the most powerful condemnations you can possibly deliver. Similarly, you can show an already-sympathetic character react with dismay and concern upon seeing or hearing about the antagonist's actions, which can also make it clear that these actions are supposed to be seen in a negative light. (Also, briefly showing something like this instead of having a character deliver a monologue on how and why it's wrong can help you avoid coming off as preachy and condescending to your audience.)

If your protagonists did anything wrong or questionable that may have helped spark an antagonist's actions, have them acknowledge it and show remorse. For example, if someone goes and joins an evil organization because the protagonists made that person feel unwanted, you might have them talk about whether they might have done things differently and whether that could have made any difference, and you might show them feeling remorseful and guilty for not having done things differently. (And remember, even if they aren't technically guilty, it's still a very natural and human thing to feel guilty at times like these.)

You can also have your protagonists (or any other sympathetic character) acknowledge that what the antagonist went through was unfortunate and undeserved. This isn't always strictly necessary, but it's still a good way to show that you consider what the antagonist went through to be deserving of sympathy, even if the antagonist's current actions are not. (EG, those who are brainwashed might choose to do terrible and unsympathetic things, but no one chooses to be brainwashed - they are essentially the victims of ideological predators.)

But make sure to acknowledge or show that whatever happened in the past, it's not a get-out-of-responsibility-free card for what's happening now. Let it also be acknowledged or shown that whatever unfair things happened to the antagonist in the past, it doesn't make the antagonist's present actions any less wrong (even if they are understandable), nor does it change the fact that the antagonist must be stopped lest others are hurt. (Whatever your antagonist went through, innocent people do not deserve to be hurt over it!)

Always remember that your characters' right to defend and protect themselves from those who would try to inflict harm on them is inviolable and non-negotiable. Someone having a tragic backstory doesn't make mistreating others any less wrong, nor does it make those who take measures to protect themselves from mistreatment selfish or cruel. Protagonists are always entitled to defend themselves against aggressive and cruel harm. Furthermore, antagonists are not wrong to defend themselves against protagonists who are inflicting excessive and needless harm on them.

Get clear on what is and isn't their faults. Check out "Is This My Character's Fault?" - A Flowchart. The last thing you want to do is make it out that your antagonists are blameless for something that they're actually at fault for. Do you plan to redeem one of your antagonists? Instead of coming up with excuses, go with the attitude that even though the antagonist did it and is at fault for it, this action doesn't have to define the rest of the antagonist's life and that there's always a choice to try to do better and learn from past mistakes.

Also remember that intentions and values matter. There's a massive difference between someone who does something bad simply because it's enjoyable or convenient, and someone who does something bad out of genuine but misguided belief that it's the only way. If your protagonists easily forgive the first type of antagonist because of a sob story, then it can look like you're using it as an excuse to get your antagonist out of being held accountable and responsible. If your protagonists refuse to give the second type a proper chance to do better, they're likely to come off as callous and/or self-righteous.

Never forget that at the end of the day, it's always on the antagonists to take responsibility or live with the consequences. This means that it's up to them not to squander any second chances, and that they are not owed forgiveness until they establish a consistent pattern of improved behavior. If they won't do this, then whatever consequences they suffer as a result are their own faults and no one else's. No one owes them sympathy or support when they choose to stay in patterns and habits that harm others.

If an antagonist doesn't get redemption, protagonists and other sympathetic characters can still show regret that things ended up turning out this way. The tragedy that the antagonist chose this path or didn't get sufficient opportunity to choose another path might be acknowledged. The necessity of doing what they had to do to stop the antagonist does not need to detract from the tragedy of the situation. (And of course, the tragedy of the situation should never detract from the necessity of doing what they have to do to stop the antagonist.)

Watch out for and avoid protagonist-centered morality. Protagonist-Centered Morality: What It Is, And How You Can Avoid It has tips to help you do this. This will help you avoid ending up with cold-hearted protagonists.

Watch out that you don't end up getting infatuated with or over-identifying with either your antagonists or your protagonists. Character Infatuation & Over-Identification - Do You Have These Problems? can help you spot signs of this and bring it into check if it happens.

And don't forget that not all antagonists are villains. An antagonist is simply someone who opposes or competes with the protagonist. It doesn't necessarily mean that the antagonist is bad or wrong. A villain is someone who acts with wanton cruelty and callousness. MORE Tips To Improve Your Villains explains the difference between villains and non-villainous antagonists in greater detail.