How to write an antagonist who feels justified

Your antagonist needs one sentence they tell themselves that turns their cruelty into a duty.

Not the reason you'd give for them. The reason they'd give, said straight, with no irony. One sentence, in their own voice, in the present tense.

That sentence makes cruelty feel like a tool instead of a failing. A belief in order becomes permission to crush anyone who disagrees. A belief in efficiency becomes permission to throw away anyone who slows things down.

It almost always has three parts. Something that really did go wrong, a new way of looking at it, and a flat denial that what they're doing is cruel.

Here's Fletcher, the music teacher in Whiplash, talking to himself: "Greatness is rare and most people are too comfortable to reach it, so I'm obligated to break every student in front of me, and if they shatter that only proves they were never great."

Keep it to one sentence. At two, they're explaining themselves instead of believing it. And if they admit they're doing wrong, you've written a confession, which is a different scene entirely.

Almost everyone has one sentence they tell themselves that makes the rest of what they do okay. They say it in the present tense and they mean it.

So write your antagonist's sentence tonight. If it makes them sound contemptible instead of uncomfortably reasonable, there's real pain under it you haven't found yet.